THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 199 



their ancestral history than is the case with embryos which undergo their develop- 

 ment within the egg." 



The most concrete critique of the law of biogenesis has been offered by Sedgwick 

 ('94). After rejecting the second proposition by showing that, while in many cases 

 the adults differ more from each other than the young, in other cases the embryos differ 

 more from each other than the adults, he takes up the main question stated in the 

 first proposition by a consideration of "The Significance of Ancestral Rudiments in 

 Embryonic Development" ('94, p. 40). It is, indeed, around this phase of the subject 

 that the discussion has centred. His views are best given by a series of excerpts from 

 his paper. Thus Sedgwick ('94, p. 41) states that "... The tendency in embryonic 

 development is to directness and abbreviation and to the omission of ancestral stages 

 of structure, and that variations do not merely affect the not-early period of life where 

 they are of immediate functional importance to the animal, but, on the contrary, that 

 they are inherent in the germ and affect more or less profoundly the whole devel- 

 opment." 



"The evidence is of this kind: (1) Organs which we know have only recently 

 disappeared are not developed at all in the embryo. For instance, the teeth of birds, 

 the fore limbs of snakes, reduced toes of bird's foot (and probably of horse's foot), 

 the reduced fingers of a bird's hand. ... (2) Organs which have (presumably) 

 recently become reduced or enlarged in the adult are also reduced or enlarged in 

 the embryo. ... (3) Organs which have been recently acquired may appear at the 

 very earliest possible stage. . . ." (p. 42). 



"The latter arrangement ["ancestral organs have disappeared without leaving 

 a trace"] seems to be the rule, the former the exception" (pp. 44-45). 



"I think it can be shown that the retention of ancestral organs by the larvae" 

 (embryos?) "after they have been lost by the adult is due to the absorption of a larva, 

 or immature free stage into embryonic life" (p. 46). A larval character thus 

 absorbed into the embryonic life, "its disappearance is no longer a matter of impor- 

 tance to the organism, because, the embryo being protected from the struggle for exist- 

 ence, the pressure of rudimentary functionless organs is unimportant to it" (p. 48). 



"Characters which disappear during free life disappear also in the embryo, but 

 characters which, though lost by the adult, are retained in the larva may ultimately 

 be absorbed into the embryonic phase and leave their traces in embryonic develop- 

 ment" (p. 49). 



"To put the matter in another and more general way. The only, functionless 

 ancestral structures which are preserved in development are those which at some 

 time or another have been of use to the organism during its development after they 



