single polypes, the cells and morphological elements. The single individuals are alike in their constitution, but are usually very different in size and development, even in those species in which the differentiation unquestionably produced by selection has not led to polymorphism or separation into personal groups performing different functions. The weal or the woe of our polypes is greatly dependent on the position which. they occupy upon the stem; the supply of nutriment primarily furnished to the single individuals is unequally and variably apportioned according to currents and tides. Hence on each polype-stem there are regions where the single polypes are especially thriving, others where they are just able to maintain themselves, others where they cannot keep their balance. But as the polype-stem is traversed by a canal system conveying the nutritive fluid and connecting the several cells, the superfluity of the well-situated cells goes to the benefit of those for whom a worse lot was prepared by their accidental position, and conversely. These relations, which, complex as they seem, are very simple for our comparison, determine the form and appearance of the polype-stem. Among a hundred thousand stems, no two will be found absolutely alike.

To return to the mutability of organisms, even if two individuals of the same species are bred under the most similar conditions imaginable, it has never been possible to pronounce them absolutely alike. That mutability is slighter in lower than in higher organisms, is a prejudice frequently repeated and fortified by the old dogma of species. The doctrine of descent and selection would fare ill if the case were

So. But as the shepherd unerringly knows the physiognomy of his sheep where an excursionist from the town sees only a general sheep's face, so to an attentive naturalist, in most of the lower organisms, the specific type resolves itself into as many varieties as individuals, irrespectively of the cases in which no specific type can be established.

As modification under given conditions, adaptation is thus as little an unknown quantity as heredity, but is merely a function of the mechanical character of mutability, or, in the widest sense of the word, of nutrition. Adaptation takes place when the organism or its parts are pliable and plastic to external influences, when they conquer and make use of them. Climate, light, humidity, nutriment, are hindrances or advantages that directly or indirectly affect the organism, and are all actively concerned in it. Surrounded by organisms, we see them without exception adapting themselves to circumstances; and if our only object is to be convinced of the formative influence of the mode of life, this is most readily done in the case of domestic animals. In his studies on the pig, H. von Nathusius, perhaps the most scientific of the celebrated breeders, shows how in the simplest cases, where the looseness of cultivated soil has facilitated the labour of grubbing, the skull of the domestic pig is arrested, by the softer structure of the cranium, at the immature form of the wild boar, and how those extreme shapes of the head in cultivated breeds, characterized by the bending and shortening of the face, and the impossibility of closing the jaw in front, are entirely the result of their altered mode of life. It is known that men, animals, and plants, removed far from their previous

abode to a new and strange environment, after a longer or shorter effort of the organism to domesticate itself, either die out, or else accommodate themselves to the ' new conditions and become acclimatized. Every acclimatization is therefore an adaptation, accompanied by modifications more or less perceptible. Thus, in consequence of the varied conditions of life, there is a wide divergence among races of men who, by their kindred language, are of the same origin, not to mention those whose relations linguistic inquiry has not yet decided. How different is the idiosyncrasy of the Englishman from that of the Hindoo! Physically and psychically, they represent two remarkable sub-races of which the peculiarities must be ascribed to adaptation,-in the latter, to a climate which requires a vegetable diet, and, eliciting neither bodily nor mental energy, favours a dreamy sensuality; in the former, to a country which is in every particular the opposite of the Indian original home. Similarly, the annual alternation in the vital phenomena of so many organisms, designated as hybernating animals, is a case of adaptation. It is changed the moment the organism is exposed to another climate, or rather acclimatization is essentially the accommodation of the hybernating animals to the new climate.

In all these examples we have the results of direct adaptation, in which the power of resistance in the individual comes into play, as does cumulative adaptation in artificial, and the survival of the fittest in natural, selection. In all cases of adaptation, one or several organs are primarily concerned, either actively or passively; and only in consequence of the resulting modifications are the other organs drawn into sym

pathy. This may be termed correlative adaptation. It might be supposed that the most perspicuous examples would be afforded by parasitic animals, in which, with the alteration of the aliment and of the alimentary apparatus, especially of the manducatory portions, is usually combined a transformation and retrogression, often extending to the total extinction of the locomotive organs, and of the entire segmentation of the body. But, although the limits are difficult to define, the cause of these associated modifications in the locomotive and alimentary apparatus consists less in their reciprocal sympathetic influence than in their simultaneous disuse.

It is, however, by correlative adaptation that, for instance, in the short-beaked races of pigeons, the middle toe and astragalus are shortened, and that in the longbeaked races these organs have shared in the elongation. In the case, however, in which short beaks are combined with short feet, a certain share in the shortening of the feet is also owing to disuse; while where the pigeonfancier took pleasure in the elongation of the beak by cumulative selection, the correlative elongation of the foot took place in spite of disuse. The most important group of correlative modifications or adaptations, always using this word in its widest acceptation, relates to the sphere of the sexes. Direct attacks on the generative organs manifest their effects on all the rest of the organism, as is best shown in animals of both sexes. castrated for the market or for labour.

We have already seen that the degree of perfection attained in the orders of the Articulata, Annulosa Vertebrata, and partially in the Radiata also, depends on the integration of the originally similar parts lying

behind or by the side of one another; hence on the division of labour. This Hacckel has designated divergent adaptation. It gives rise to the remarkable polymorphism, which appears especially in the marvellous forms of the Hydra tuba; and, higher up, in the segmentation of the classes of the Termites and the Bee, &c.

So far as modification coincides with adaptation, the direct adaptations hitherto discussed may be opposed by a series of indirect adaptations. Among these may be comprised a series of phenomena of which the causes do not fall within the life of the individual, but are to be sought in influences by which the parents were affected. It is obvious that we here come into contact with the province of heredity in a manner well known to breeders. Thus H. v. Nathusius, in his studies on the formation of the pig's skull," says :-" From the facts here collected, it is plain that the transmission, the transfer of the form of head from the parents to the offspring, does not unconditionally ensue. If the form of skull, which we will briefly term the cultivated form, be the product of nutrition and mode of life, hence of external influence,-if it can be differently formed in the same individual, and is therefore not constant,-in that case the heredity of this form cannot be spoken of without qualification. The form itself will not be transmitted to the offspring, but only the tendency to the form. This may be inferred from the circumstance that from generation to generation, and to a certain degree, the form increases in peculiarity. If we rear a common with a thoroughbred pig, and if we allow exactly the same influences of nutriment and keeping