« ПредыдущаяПродолжить »
duction of protoplasm, that primordial organism, from the atoms of its constituents.
Hence the beginning of life at some bygone period is likewise not susceptible of demonstration; but the commencement of animate being at some definite era of development is a logical necessity, and by no means a vulnerable point in the doctrine of Descent."4
We have already incidentally mentioned a man who, although not so eminent as Darwin, has the glory of having independently discovered the law of natural selection, and of having, after Darwin had come forward with his fundamental work, supported the theory of selection by a profusion of original observations. This is Alfred Russell Wallace." In a paper, published in 1855, he demonstrated the dependence of the flora and fauna on the geographical position and geological nature of the district of propagation, and the close connection of the species, according to time and habitat, with kindred species previously existing; and in a second work, in the year 1858, on the inclination of varieties to deviate without limit from the original type, we find a disquisition on the importance of the struggle for existence, the consequences of adaptation, the selection of the most useful, and the replacement of the earlier species by the establishment of the more valuable varieties. We shall repeatedly have occasion to draw upon the rich supplies of his researches.
Heredity-Reversion-Variability-Adaptation-Results of Use and Disuse of Organs--Differentiation leading to Perfection.
THE two properties of organic being which determine and regulate the relation of the offspring to the progenitors, and which not only assign to individuals their position in the surrounding world, but also help them to attain it, are transmission or heredity, and adaptation.
Heredity is the conservative, adaptation, the progressive principle. Yet all heredity is not directed to immutability, and many cases of adaptation involve morphological and physiological retrogression. For the elucidation of the inherited peculiarities of organisms, we reconstruct their pedigree; by the characters acquired by adaptation, we test the pliability of organisms in the lapse of time, and trace the ramifications of the pedigree. Groups of organisms, in which the conservative principle predominates, certainly evince their powers of endurance in the struggle for existence, but they make no advance in physiological value, and are outstripped by the more progressive groups which yield to obstacles and profit by them, a course of which human life also affords so many examples.
As the phenomena of heredity are usually more obvious than the results of adaptation, the latter was almost
entirely neglected by naturalists in former days. And indeed what comparison in organic nature can be made so frequently and universally as the resemblance of the offspring to the parent? An anatomist, it is true, quaintly attempted to work out the proposition that the resemblance in the children is not dependent on heredity, but is the result of identical and similar influences, customs, and habits, prevalent in families. But this paradoxical theory requires no special refutation. It is quite true that similar habits and similar external impulses elicit a certain similarity of demeanour and appearance; but if the little son of the pompous millionaire apes his father, it cannot be said that he has likewise mimicked his large or small nose, &c., or has acquired it by a similar call for adaptation. We have only cursorily alluded to this quibble, in flagrant contradiction as it is with every experience; and, in conformity with general opinion, we corroborate the transmission of the parental characteristics to the offspring. The breeders of animals in particular has occasion to observe these transmissions specially, and to evolve their astounding progress from the combination and reciprocal influence of the various forms and degrees of heredity.
It is well known that not only are normal conditions transmitted, but monstrosities are also reproduced through several generations, and, as we have seen in the instance of the crook-legged sheep of Massachusetts, may even be established as the characters of a race. A mere reference to the inheritability of morbid tendencies, bodily and mental, will enable us to realize this intrinsic connection of the offspring to the ancestors. Only since the theory of selection has rendered the modalities of the transmis
sion of bodily characters a subject of more profound study, have general and national psychology been impelled to estimate the influence of heredity in the province of the mind, and demonstrate how, in the various races and families of nations, the molecular peculiarities of the brain, the tendency of character and intelligence of the individuals, and whole series of ideas, conform both in vigour and purport to the laws of heredity.
It is manifest that the key to the phenomena of heredity must be looked for in the process of reproduction. The molecular motions and disturbances, the inconceivably minute mechanical transfers which take place, do not, indeed, admit of observation. They are, however, no more "obscure" and "enigmatical," as they are so readily termed, than the invisible, but not supernatural motions, on the control and calculation of which the stately edifice of theoretic Chemistry and Physics securely rests. With the advance from asexual to sexual reproduction, and from the simple to the more perfect organisms, the difficulty of representation increases, but not that of abstract comprehension. If a low organism, a monad, divides itself, the divided individuals differ from the parent individual only in their inferior bulk, and the difference of their functions is, as to quality, nil.
So, too, where gemmules and germs separate from a parent organism, the dower of the offspring is so large that identity in form and function of progenitor and progeny appears self-evident and natural. But the sexual reproduction of composite organisms is, as we have known since the old doctrine of the aura seminalis was refuted, also a separation of material portions of the parental organisms. It is still a mechanical process which
is not incomprehensible, and seems inexplicable only if we make the naturally futile attempt to bring sensibly before us the infinitely minute agencies which operate both mechanically and chemically. In the "Variation of Plants and Animals," Darwin has set up a provisional hypothesis of Pangenesis. He says that all phenomena of heredity and reversion would thereby be rendered possible, that in every elementary or cellular portion of the organism innumerable gemmules are produced, which are hoarded up in the reproductive elements, in every ovum, in every sperm corpuscule, and might remain latent during hundreds of gencrations, and only then exhibit their powers in reversion.56 This This hypothesis, it appears, has met with no ready approbation, probably, as it seems to us, because, in the attempt to meditate upon it, the sensible representation forces itself forward only to prove inadequate. But if it be steadfastly borne in mind that in Protoplasm, as Rollet" appropriately terms it, the most complex phenomenal forms of life possess a most persistent witness of their connection with the simplest, it follows that the general laws shown to be true or probable with reference to the simplest organisms, must be applicable to the most perfect also. This holds good also in reproduction, which, in its fundamental phenomena, offers nothing that cannot be based upon molecular physics applied to colloidal living substance capable of imbibition, and thus divested of vitalistic dualism.
The more highly complex is an organism, that is, the greater the differentiation in the development from the protoplasm of the germ-cell to maturity, the more heterogeneously does heredity display itself. These