STRUCTURE AND DEVELOPMENT 65 



from the common mode show where the several divi- 

 sions took leave of one another in their evolution, 

 a point that comes out with great clearness when the 

 facts of mammalian development are broadly com- 

 pared. The embryos of carnivora and rodents and 

 hoofed animals are alike in their earlier develop- 

 ment, and their agreement means a community of 

 origin. At a certain point the cat and dog depart 

 from the common mode, but they remain alike up to a 

 far later stage than the one in which they are similar 

 to the embryos of rats and sheep. The rat and 

 squirrel and rabbit, on their part, remain together 

 until long after they take leave of the carnivora and 

 ungulates; while the sheep and cattle and pigs have 

 their own branch line, which they follow in company 

 after leaving the embryos of the other orders. The 

 reasons for these facts seem to be that the members 

 of the three orders exemplified have evolved from the 

 same stock, which accounts for their embryonic simi- 

 larity for a long time after they collectively come to 

 differ from amphibia and reptiles, while the members 

 in each order became differentiated only later, where- 

 fore their embryonic paths coincide for a longer period. 

 Thus the degree of adult resemblance which indicates 

 the closeness of relationship corresponds with the de- 

 gree of embryonic agreement ; that is, the cat and dog 

 are much alike and their modes of development are 

 essentially the same to the latest stages, while the cat 

 and horse agree only during the earliest and middle 

 stages, and their lines diverge before those of the cat 

 and dog on the one hand, or those of the horse and 

 pig on the other. 



