THE ORIGIN OF THE HUMAN BODY 283 



tion of the more complex structures of adult life, and to im- 

 provise simpler substitutes for use until such a time as they 

 have accumulated a sufficient reserve of energy and materials 

 to complete the work of their more elaborate adult organiza- 

 tion. The young starfish, for example, arising as it does from 

 an egg but scantily supplied with yolk, is forced, from the very 

 outset, to shift for itself, in coping with the food-getting 

 problem. Under stress of this necessity, it economizes its 

 slender resources by constructing the extremely simple diges- 

 tive and motor apparatus characteristic of the larva in its 

 bilaterally-symmetrical Bipinnaria stage, and postponing the 

 development of the radially-symmetrical structure character- 

 istic of the adult stage, until it has stored up the wherewithal 

 to complete its metamorphosis. 



From this viewpoint, there is no difficulty in understanding 

 why temporary solutions of the excretory problem should 

 precede the definitive solution of this problem in mammalian 

 embryos. The problem of excretion is urgent from the outset, 

 and its demands increase with the growth of the embryo. 

 It is only natural, then, that a series of improvised structures 

 should be resorted to, in a case of this kind; and, since these 

 temporary solutions of the excretory problem must, of neces- 

 sity, be as simple as possible, it should not be in the least 

 surprising to find them coinciding with the permanent solu- 

 tions adopted by inferior organisms less complexly organized 

 than the mammals. Hence the bare fact of resemblance be- 

 tween the transitory embryonic kidney of a mammal and the 

 permanent adult kidney of a fish would have no atavistic 

 significance. We know of innumerable cases in which an 

 identical adaptation occurs in genetically unrelated organisms. 

 The cephalopod mollusc Nautilics, for example, solves the 

 problem of light-perception in the identical manner in which 

 it is solved by the vertebrates. This mollusc has the perfect 

 vertebrate type of eye, including the lens and all other parts 

 down to the minutest detail. The fact, however, that the 

 mollusc solves its problem by using the stereotyped solution 



