84 ESSAY ON CLASSIFICATION 



phibia and Reptiles proper, so long united as one class, constitute 

 two distinct classes. In the main, the development of the true Reptiles 

 agrees very closely with that of the Birds, while the Amphibians 

 resemble more the true fishes. In no class are renewed embryological 

 investigations, extending over a variety of families, so much needed, 

 as in that of Birds, though the general development of these animals 

 is, perhaps, better known than that of any other type; while the 

 class of Mammalia has found in Bischoff a most successful and thor- 

 ough investigator. 



Embryology has, however, a wider scope than to trace the growth 

 of individual animals, the gradual building up of their body, the 

 formation of their organs, and all the changes they undergo in their 

 structure and in their form; it ought also to embrace a comparison 

 of these forms and the successive steps of these changes between all 

 the types of the animal kingdom, in order to furnish definite stand- 

 ards of their relative standing, of their affinities, of the correspond- 

 ence of their organs in all their parts. Embryologists have thus far 

 considered too exclusively the gradual transformation of the egg into 

 a perfect animal; there remains still a wide field of investigation 

 to ascertain the different degrees of similarity between the successive 

 forms an animal assumes until it has completed its growth and the 

 various forms of different kinds of full-grown animals of the same 

 type; between the different stages of complication of their structure 

 in general and the perfect structure of their kindred; between the 

 successive steps in the formation of all their parts and the various 

 degrees of perfection of the parts of other gxoups; between the normal 

 course of the whole development of one type compared with that of 

 other types, as well as between the ultimate histological differences 

 which all exhibit within certain limits. Though important fragments 

 have been contributed upon these different points, I know how 

 much remains to be done, from the little I have as yet been able 

 to gather myself by systematic research in this direction. 



I satisfied myself long ago that Embryology furnishes the most 

 trustworthy standard to determine the relative rank among animals. 

 A careful comparison of the successive stages of development of the 

 higher Batrachians furnishes perhaps the most striking example of 

 the importance of such investigations. The earlier stages of the Tad- 



