1 64 E VOL UTION OF TO-DA Y. 



ance in various embryos of the characteristics which 

 mark it as belonging to one of the various types. A 

 rudimentary nervous system appears, indicating the 

 vertebrate type ; a peculiar mesoderm indicates the 

 echinoderm sub-kingdom, etc. Next, it is seen that, 

 having general characteristics, the embryo assumes 

 those of the class to which it belongs. The verte- 

 brate ceases to be a vertebrate in general, and be- 

 comes a fish or mammal ; the echinoderm becomes 

 a star-fish or sea-urchin. Next follow the charac- 

 ters of the order the sub-order, and, as develop- 

 ment proceeds, the family, and finally the genus and 

 species, the last distinction frequently not appearing 

 until the fully adult condition is reached. 



Embryology, therefore, forms the final link in the 

 chain of evidence which proves that there is a funda- 

 mental unity in the organic world, for the fact that 

 embryology repeats past history proves that there is 

 some unitary bond uniting the present with the past. 

 Evolution offers heredity and descent as an explan- 

 ation of this bond, and although this is not the only 

 possible explanation, it is certainly the simplest, and 

 the only one yet offered which is founded on natural 

 law ; and while with advancing knowledge the evi- 

 dence is growing more complicated and difficult to 

 understand, there cannot be the slightest doubt that 

 it is becoming continually more and more clear that 

 this explanation is adequate to meet the facts. 



