470 DR. ST. GEORGE M1VART ON THE DEVELOPMENT [Julie 17, 



very different forms has been shown by Professor Lankester in the 

 very interesting case of Bacterium rufescens\ 



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 modifiability, 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 abso- 

 lutely impossible. 



The foregoing considerations tend to show that every variation is 

 a function 2 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 environ- 

 ment, 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 clue to descent 3 , but which demon- 

 strate the existence of an intra-orgamc activity, the laws of which 

 have yet to be investigated. Comparative anatomy, pathology, aud 

 teratology combine to point out the action of this internal force. 



As to its action as exemplified in the homloogies of the Crustacea 

 Mr. Brooks 4 makes the following remarks :— 



" Special homology may be defined in two ways, morphologically 

 and pbylogenetically. 



" 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 phyiogenetic point of view it is a resemblance which is 

 due to community of origin or heredity from a common ancestor. . . . 



" Now are the phenomena of serial and lateral homology like those 

 of special homology in this second or phyiogenetic sense, as well as 

 in a morphological sense? 



" On 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 

 phyiogenetic standpoint ; and if we assume that this series was at 



1 See ' Quarterly Journal of Microsc. Sci.' new series (1873), vol. xiii. p. 408, 

 and vol. xvi. (1S70), 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- 

 mobranclis, Trans. Zool. Hoc. 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. 



