LAWS OF DEVELOPMENT 125 



If we consider now development within the type, which 

 is the only legitimate thing to do, we arrive at certain laws 

 governing the relation of embryos to one another. For 

 instance, at a certain stage vertebrate embryos are un- 

 commonly alike. Von Baer had two in spirit which he was 

 unable to assign to their class among amniotes ; they might 

 have been lizard, bird, or mammal, he could not say definitely 

 which. 1 Generally the farther back we go in the develop- 

 ment of Vertebrates the more alike we find the embryos. 

 The type-characters are first to appear, then the class 

 characters, then the characters distinguishing the lesser 

 classificatory groups. "From a more general type the 

 special gradually emerges" (p. 221). The chick is first a 

 Vertebrate, then a land-vertebrate, then a bird, then a land- 

 bird, then a gallinaceous bird, and finally Gallus domestic/is. 

 Development within the type is a progress from the general 

 to the special, a real evolution. The more divergent two 

 adults are, the farther back we must go in their development 

 to find an agreement between their embryos. We can sum 

 up the case in the following laws : 



"(i) That tJie general characters of the Ing group to 

 which the embryo belongs appear in development earlier than 

 the special characters. In agreement with this is the fact 

 that the vesicular form is the most general form of all ; for 

 what is common in a greater degree to all animals than the 

 opposition of an internal and an external surface ? 



" (2) The less general structural relations are formed 

 after t/ie more general, and so on until the most special 

 appear. 



"(3) The embryo of any given form, instead of passing 

 through the state of other definite forms, on the contrary separates 

 itself from them. 



" (4) Fundamentally the embryo of a higher animal form 



1 Compare a parallel passage in Prevost et Dumas : "At the very 

 first sight one will be struck with the resemblance between the forms 

 of the very early embryos of these two classes, a resemblance so extra- 

 ordinary that one cannot refuse to admit the conclusions resulting from 

 it. The resemblance is so striking that one can defy the most experienced 

 observer to distinguish in any way the embryos of dog or rabbit . . . 

 from those of fowls or clucks of a corresponding age." Ann. Set. nat., 

 iii., p. 132, 1824. 



