220 Transactions of tue American Institute. 



Thus tlie lowest group in the auimal kingdom, tlie protozoa are all 

 aquatic. Two next lowest branch, the radiata are all aquatic ; raol- 

 lusca comes next, and with few exceptions, e. g., the land snails are 

 all aquatic, and even these occupy damp and wet places as a necessity 

 of their existence. In the next brancli, the articulates, while its 

 highest class, the insects are terrestrial, or «rial, the two lowest classes 

 the worms and Crustacea, with few exceptions are aquatic. Now if 

 wc consider the vertebrata, we shall find its lowest class, the fishes, are 

 all aquatic. The batrachians are amphibious and aquatic. The 

 reptiles have certain groups that are aquatic, and though the two 

 highest classes of the vertebrates, the birds and mammals, are ter- 

 restrial, j^et, their lowest forms, the auks, of the birds, and the whales, 

 and dolphins, of the mammals, are aquatic. 



Another principle of far more special application is that based 

 upon embryological data. It has been found that animals, in their 

 development, pass through certain stages that recall the adult condi- 

 tions of animals below them. A common example may be citei 

 among the batrachians, where the lowest forms resemble fishes hav- 

 ing tufts of gills on the sides of the neck and a long finned tail. A 

 little higher up, we conic to those that have the same general form, 

 but the gills are wanting, and they breathe air. The toads and frogs 

 are the highest, and here the tail is absent, and now locomotion is 

 performed by the strongly developed hind legs. If we now examine 

 the development of any frog, it will be found that, on leaving the 

 a^^g, the animal is without legs, and swims with its tail, and breathes 

 by gills ; that by successive steps it becomes an air-breather, little 

 legs bud out, and ultinuitely the tail is abso^rbed, and we have the 

 conqileted animal. You will see by these figures that the difierent 

 stages resemble some of the forms I have just drawn. Many other 

 examples might be cited, but time will not allow me to present 

 thom. 



This principle has been recognized under a variet}' of propositions 

 by many minds. Goethe, in 1807, and Von Baer in 1828, showed 

 tliat development was always from the general to the special, from 

 the liomogeneous to the heterogeneous, from tlie sinq.ile to the com- 

 plex, and this by a gradual series of dififercntiation, and Herbert 

 Spenser has applied the same law of evolution in many new and 

 startling ways. And as a law of evolution, it is interesting to notice 

 that in the advent of animal life upon earth we have a sequence of 

 forms that illustrate the fact that the earlier forms created within 



