THE GASTRJ3A THEORY. 11 



embryological characters are of high value as guides in 

 classification, and it may, I think, be regarded as well- 

 established that, just as the contents and sequence of 

 rocks teach us the past history of the earth, so is the 

 gradual development of the species indicated by the 

 structure of the embryo and its developmental changes. 



"When the supporters of Darwin are told that his 

 theory is incredible, they may fairly ae-k why it is im- 

 possible that a species in the course of hundreds of 

 thousands of years should have passed through changes 

 which occupy only a few days or weeks in the life- 

 history of each individual ? 



The phenomena of yolk- segmentation, first observed 

 by Prevost and Dumas, are now known to be, in some form 

 or other, invariably the precursors of embryonic develop- 

 ment ; while they reproduce, as the first stages in the 

 formation of the higher animals, the main and essential 

 features in the life-history of the lowest forms. The 

 ' blastoderm,' as it is called, or first germ of the embryo 

 in the egg, divides itself into two layers, corresponding, 

 as Huxley has shown, to the two layers into which the 

 body of the Coelenterata may be divided. Not only so, 

 but most embryos at an early stage of development have 

 the form of a cup, the walls of which are formed by the 

 two layers of the blastoderm. Kowalevsky was the first 

 to show the prevalence of this embryonic form, and sub- 

 sequently Lankester and Haeckel put forward the hypo- 

 thesis that it was the embryonic repetition of an 

 ancestral type, from which all the higher forms are 

 descended. The cavity of the cup is supposed to be the 

 stomach of this simple organism, and the opening of the 

 cup the mouth. The inner layer of the wall of the cup 



