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Show 470 DR. ST. GEORGE MIVART ON THE DEVELOPMENT [June 17, very different forms has been shown by Professor Lankester in the very interesting case of Bacterium rufescens'1. It is also obvious that the very same influences (e. g. amounts of light, heat, moisture, &c.) will produce different effects in different species, as also that the nature of some species is more stubborn and less prone to variation than that of others. Such for example is the case with the Ass, the Guinea-fowl, and the Goose as compared with the Dog, the Horse, the Domestic Fowl, and the Pigeon. Thus both the amount and the kind of variability differ in different races, and such constitutional capacities, or incapacities, tend to be inherited by their derivative forms, and so every kind of animal must have its own inherent powers of inodifiability, or resistance, so that no organism or race of organisms can vary in an absolutely indefinite manner ; aud if so, then unlimited variability must be a thing absolutely impossible. The foregoing considerations tend to show that every variation is a function2 of "heredity" and "external influence"-i.e. is the result of the reaction of the special nature of each organism upon the stimuli of its environment. In addition to the action of heredity and the action of the environment, there is also a peculiar kind of action due to an internal force which has brought about so many interesting cases of serial and lateral homology which cannot be due to descent3, but which demonstrate the existence of an intra-organic activity, the laws of which have yet to be investigated. Comparative anatomy, pathology, and teratology combine to point out the action of this internal force. As to its action as exemplified in the homloogies of the Crustacea Mr. Brooks4 makes the following remarks :- " Special homology may be defined in two ways, morphologically and phylogenetically. " From the morphological point of view an homology is a similarity in essential plan of structure, which may be obscured by differences due to diversity of function. " From the phylogenetic point of view it is a resemblance which is due to community of origin or heredity from a common ancestor. . . . " N o w are the phenomena of serial and lateral homology like those of special homology in this second or phylogenetic sense, as well as in a morphological sense ? " O n the assumption that the remote ancestor of the Crustacea was a community of independent organisms, all of which had inherited their organization from the same parent, we might answer that serial homology is like special homology when viewed from a phylogenetic standpoint; and if we assume that this series was at 1 See ' Quarterly Journal of Microsc. Sci.' new series (1873), vol. xiii. and vol. xvi. (1876), p. 27. 2 In the mathematical sense of the word. 3 Such e. g. as some of those noticed by me in a paper on the Fins of Elas-mobranchs, Trans. Zool. Soc. vol. x. p. 439. 4 W . K. Brooks in Phil. Trans. 1882 ; 'A Study of Morphology,' p, 57 ; and Serial Homology and Bilateral Symmetry in Crustacea,' p. 125. ' |