EMBRYOLOGY AND HYPOTHETICAL HISTORY. I2/ 



cording to the descent theory, have had precisely 

 the same history until comparatively recent times. 

 Genera of the same family separated from each 

 other at an earlier period, and families of the same 

 order at still more remote times, while the different 

 sub-kingdoms have had no common history since 

 the earliest ages. If, now, embryology were an 

 exact repetition of past history, we should expect 

 to find the development of individuals of related 

 species agreeing except in the very latest stages. 

 Genera would show a similarity of development not 

 quite so long, families and orders would separate 

 from each other still earlier, while the sub-kingdoms 

 would show no similarity except in the earliest 

 stages. Every two animals should begin their de- 

 velopment alike, and the point at which they become 

 unlike would depend upon the closeness of their 

 genetic relation. 



Around this point has centred a large part of 

 modern embryological research. Not always, per- 

 haps, realizing the significance of their results, 

 naturalists have been trying to make the various 

 facts collected harmonize with each other. For it 

 is almost immediately recognized, was indeed recog- 

 nized by Spencer and Haeckel, that the embryo- 

 logical history of animals is not in strict harmony 

 with their relations to each other. Two animals 

 quite closely related in adult anatomy, may 

 have embryological histories differing very widely 

 from each other from the very first stages. The 

 embryology of the genera of a single family does 

 not always represent a unity, although as a rule it 



