XVII] THE COMPARISON OF RELATED FORMS 727 



separate comparisons, and our general method will be found 

 inapplicable. Now precisely this independent variability of parts 

 and organs — here, there, and everywhere within the organism 

 — would appear to be implicit in our ordinary accepted notions 

 regarding variation; and, unless I am greatly mistaken, it is 

 precisely on such a conception of the easy, frequent, and normal 

 independent variability of parts that our conception of the process 

 of natural selection is fundamentally based. For the morphologist, 

 when comparing one organism with another, describes the 

 differences between them point by point, and "character" by 

 "character*." If he is from time to time constrained to admit 

 the existence of "correlation" between characters (as a hundred 

 years ago Cuvier first showed the way), yet all the while he 

 recognises this fact of correlation somewhat vaguely, as a pheno- 

 menon due to causes which, except in rare instances, he can hardly 

 hope to trace ; and he falls readily into the habit of thinking and 

 talking of evolution as though it had proceeded on the lines of his 

 own descriptions, point by point, and character by characterf. 



But if, on the other hand, diverse and dissimilar fishes can be 

 referred as a whole to identical functions of very different co- 

 ordinate systems, this fact will of itself constitute a proof that 

 variation has proceeded on definite and orderly lines, that a 

 comprehensive "law of growth" has pervaded the whole structure 

 in its integrity, and that some more or less simple and recognis- 

 able system of forces has been at work. It will not only show 

 how real and deep-seated is the phenomenon of "correlation," 

 in regard to form, but it will also demonstrate the fact that 

 a correlation which had seemed too complex for analysis or 



* Cf. supra, p. 714. 



f Cf. Osborn, H. F., On the Origin of Single Characters, as observed in fossil 

 and living Animals and Plants, Amer. Nat. xlix, pp. 193-239, 1915 (and other 

 papers); ibid. p. 194, "Each individual is composed of a vast number of somewhat 

 similar new or old characters, each character has its independent and separate 

 history, each character is in a certain stage of evolution, each character is correlated 

 with the other characters of the individual... .The real problem has always been 

 that of the origin and development of characters. Since the Origin of Species 

 appeared, the terms variation and variability have always referred to single 

 characters; if a species is said to be variable, wo mean that a considerable number 

 of the single characters or groups of characters of which it is composed are variable," 

 etc. 



