408 NOTES. 



in the embryo of every vertebrate animal, show that we here have to 

 deal with an hereditary character, and its remarkable persistence and 

 constancy are evidently due to the fact that its presence is essentially 

 bound up with the development of an organ so wonderfully fitted for 

 the most various morphological and physiological differentiation as 

 the vertebral column (and skeleton) of the Vertebrata. But, working 

 backwards, in the series of invertebrate animals we first lose sight of 

 the skeleton, and presently even the chorda dorsalis disappears. 



From this it would seem that such an axial cord must have appeared 

 at first, once or even more than once, in some group of invertebrate 

 animals, i.e., was elaborated from cells already existing in other forms. 

 This primitive chorda must from our point of view have had some defi- 

 nite function, and from everything that we know of the histology of 

 the cells of the chorda, we must regard it, even in its simplest form, 

 as an elastic prop or fulcrum for the movements of the animal; conse- 

 quently the primitive chorda was an organ that derived its fitness to 

 exist from its adaptation (and consequent modification) to the function 

 of affording support to the whole animal structure. Thus in the first 

 instance it can only have had the value of a character of adaptation. 

 Moreover, it must inevitably have preserved this value only, if it had 

 not contained, in itself, and through its influence on the other organs 

 connected with it, the elements of the most varied differentiation into 

 numerous dissimilar forms. It is only by the fact that it and the 

 tissues immediately surrounding it were in the highest degree plastic, 

 that it acquired its value as an hereditary character. An accurate 

 analysis of each separate organ will bring us ultimately to a stage in 

 which its existence seems to be wholly dependent on its special adapta- 

 tion to some definite purpose, or to some condition of existence. 



Note 3, page 1 7. Rudimentary organs are extraordinarily numerous, 

 and occur in a more or less significant form in most animals. Their 

 high theoretical importance has been sufficiently indicated by Darwin, 

 to whose works the reader is referred. Their most essential peculiarity 

 is their incapacity for fulfilling the functions for which, by their 

 structure and position, they would seem exclusively intended. The 

 question, however, still remains, as Leuckart has pointed out, as to 

 whether we are justified in saying that such rudimentary organs are in 

 fact wholly useless. The teeth of the Vertebrata are, as we know, used 

 only for biting and masticating, or as weapons. The male dugong does, 

 in fact, so use bis tusks, as is shown by the invariably worn condition of 

 the point, on the external surface of each. The female has equally large, 

 nay, even larger tusks ; but they are not used at any rate, not in the 

 same way. But it seems not improbable that, merely by their great weight, 

 they may assist in certain movements of the head, for instance, in grazing 

 on sea-weeds ; and in this respect they may actually have acquired a 

 physiological significance without losing their character as rudimentary 



