at all events had an origin quite distinct from that of varieties, this law could have no exceptions, because, if any two species had been found to be fertile when crossed and their hybrid offspring to be also fertile, this fact would have been held to prove them to be not species but varieties. On the other hand, if two varieties had been found to be infertile, or their mongrel offspring to be sterile, then it would have been said: These are not varieties but true species. Thus the old theory led to inevitable reasoning in a circle; and what might be only a rather common fact was elevated into a law which had no exceptions.

The elaborate and careful examination of the whole subject by Mr. Darwin, who has brought together a vast mass of evidence from the experience of agriculturists and horti culturists, as well as from scientific experimenters, has demonstrated that there is no such fixed law in nature as was formerly supposed. He shows us that crosses between some varieties are infertile or even sterile, while crosses between some species are quite fertile; and that there are besides a number of curious phenomena connected with the subject which render it impossible to believe that sterility is anything more than an incidental property of species, due to the extreme delicacy and susceptibility of the reproductive powers, and dependent on physiological causes we have not yet been able to trace. Nevertheless, the fact remains that most species which have hitherto been crossed produce sterile hybrids, as in the well-known case of the mule; while almost all domestic varieties, when crossed, produce offspring which are perfectly fertile among themselves. I will now endeavour to give such a sketch of the subject as may enable the reader to see something of the complexity of the problem, referring him to Mr. Darwin's works for fuller details.

Extreme Susceptibility of the Reproductive Functions.

One of the most interesting facts, as showing how susceptible to changed conditions or to slight constitutional changes are the reproductive powers of animals, is the very general difficulty of getting those which are kept in confinement to breed; and this is frequently the only bar to domesticating wild species. Thus, elephants, bears, foxes,

and numbers of species of rodents, very rarely breed in confinement; while other species do so more or less freely. Hawks, vultures, and owls hardly ever breed in confinement ; neither did the falcons kept for hawking ever breed. Of the numerous small seed-eating birds kept in aviaries, hardly any breed, neither do parrots. Gallinaceous birds usually breed freely in confinement, but some do not; and even the guans and curassows, kept tame by the South American Indians, never breed. This shows that change of climate has nothing to do with the phenomenon; and, in fact, the same species that refuse to breed in Europe do so, in almost every case, when tamed or confined in their native countries. This inability to reproduce is not due to ill-health, since many of these creatures are perfectly vigorous and live very long.

With our true domestic animals, on the other hand, fertility is perfect, and is very little affected by changed conditions. Thus, we see the common fowl, a native of tropical India, living and multiplying in almost every part of the world; and the same is the case with our cattle, sheep, and goats, our dogs and horses, and especially with domestic pigeons. It therefore seems probable, that this facility for breeding under changed conditions was an original property of the species which man has domesticated a property which, more than any other, enabled him to domesticate them. Yet, even with these, there is evidence that great changes of conditions affect the fertility. In the hot valleys of the Andes sheep are less fertile; while geese taken to the high plateau of Bogota were at first almost sterile, but after some generations recovered their fertility. These and many

other facts seem to show that, with the majority of animals, even a slight change of conditions may produce infertility or sterility; and also that after a time, when the animal has become thoroughly acclimatised, as it were, to the new conditions, the infertility is in some cases diminished or altogether ceases. It is stated by Bechstein that the canary was long infertile, and it is only of late years that good breeding birds have become common; but in this case no doubt selection has aided the change.

As showing that these phenomena depend on deep-seated causes and are of a very general nature, it is interesting

to note that they occur also in the vegetable kingdom. Allowing for all the circumstances which are known to prevent the production of seed, such as too great luxuriance. of foliage, too little or too much heat, or the absence of insects to cross-fertilise the flowers, Mr. Darwin shows that many species which grow and flower with us, apparently in perfect health, yet never produce seed. Other plants are affected by very slight changes of conditions, producing seed freely in one soil and not in another, though apparently growing equally well in both; while, in some cases, a difference of position even in the same garden produces a similar result.1

Reciprocal Crosses.

Another indication of the extreme delicacy of the adjustment between the sexes, which is necessary to produce fertility, is afforded by the behaviour of many species and varieties when reciprocally crossed. This will be best illustrated by a few of the examples furnished us by Mr. Darwin. The two distinct species of plants, Mirabilis jalapa and M. longiflora, can be easily crossed, and will produce healthy and fertile hybrids when the pollen of the latter is applied to the stigma of the former plant. But the same experimenter, Kölreuter, tried in vain, more than two hundred times during eight years, to cross them by applying the pollen of M. jalapa to the stigma of M. longiflora. In other cases two plants are so closely allied that some botanists class them as varieties (as with Matthiola annua and M. glabra), and yet there is the same great difference in the result when they are reciprocally crossed.

Individual Differences in respect to Cross-Fertilisation.

A still more remarkable illustration of the delicate balance of organisation needful for reproduction, is afforded by the individual differences of animals and plants, as regards both their power of intercrossing with other individuals or other species, and the fertility of the offspring thus produced. Among domestic animals, Darwin states that it is by no means rare to find certain males and females which will not breed 1 Darwin's Animals and Plants under Domestication, vol. ii. pp. 163-170.

together, though both are known to be perfectly fertile with other males and females. Cases of this kind have occurred among horses, cattle, pigs, dogs, and pigeons; and the experiment has been tried so frequently that there can be no doubt of the fact. Professor G. J. Romanes states that he has a number of additional cases of this individual incompatibility, or of absolute sterility, between two individuals, each of which is perfectly fertile with other individuals.

During the numerous experiments that have been made on the hybridisation of plants similar peculiarities have been noticed, some individuals being capable, others incapable, of being crossed with a distinct species. The same individual peculiarities are found in varieties, species, and genera. Kölreuter crossed five varieties of the common tobacco (Nicotiana tabacum) with a distinct species, Nicotiana glutinosa, and they all yielded very sterile hybrids; but those raised from one variety were less sterile, in all the experiments, than the hybrids from the four other varieties. Again, most of the species of the genus Nicotiana have been crossed, and freely produce hybrids; but one species, N. acuminata, not particularly distinct from the others, could neither fertilise, nor be fertilised by, any of the eight other species experimented on. Among genera we find somesuch as Hippeastrum, Crinum, Calceolaria, Dianthus-almost all the species of which will fertilise other species and produce hybrid offspring; while other allied genera, as Zephyranthes and Silene, notwithstanding the most persevering efforts, have not produced a single hybrid even between the most closely allied species.

Dimorphism and Trimorphism.

Peculiarities in the reproductive system affecting individuals of the same species reach their maximum in what are called heterostyled, or dimorphic and trimorphic flowers, the phenomena presented by which form one of the most remarkable of Mr. Darwin's many discoveries. Our common cowslip and primrose, as well as many other species of the genus Primula, have two kinds of flowers in about equal proportions. In one kind the stamens are short, being situated about the middle of the tube of the corolla, while the

style is long, the globular stigma appearing just in the centre of the open flower. In the other kind the stamens are long, appearing in the centre or throat of the flower, while the style is short, the stigma being situated halfway down the tube at the same level as the stamens in the other form. These two forms have long been known to florists as the 'pin-eyed" and the "thrum-eyed," but they are called by Darwin the long-styled and short-styled forms (see woodcut).

The meaning and use of these different forms was quite unknown till Darwin discovered, first, that cowslips and primroses are absolutely barren if insects are prevented from visiting them, and then, what is still more extraordinary, that each form is almost sterile when fertilised by its own pollen, and comparatively infertile when crossed with any other plant of its own form, but is perfectly fertile when the pollen of a long-styled is carried to the stigma of a short-styled plant, or vice versa. It will be seen, by the figures, that the arrangement is such that a bee visiting the flowers will carry the pollen from the long anthers of the short-styled form to the stigma of the long-styled form, while it would never reach the stigma of another plant of the short-styled form.