THE GA5TBLSA THEORY. 11 



embry ological characters are of high value as guides in 

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

 established that, just as the contents and srqotnre of 

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

 gradual development of the species indicated by Ae 

 structure of the embryo and its developmental changes. 



When the supporters of Darwin are told that Ins 

 theory is incredible, they may fairly ask why it is im- 

 possible that a species in the coarse of hundreds of 

 thousands of years should have passed through changes 

 which occupy only a lew days or weeks in the fife- 

 history of each individual ? 



The pht-MJimgiMA of yolk segmentation, first observed 

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

 or other, invariably the precursors of emhtyonic develop- 

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

 formation of die higher animals^ the main and essential 

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

 'blastoderm, 7 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 Ccelenterata may be divided. Xot only go, 



but most embrvosat an earivsta^e of development have 



j i 



the form of a cup, the walk of which are formed by die 

 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 die higher forms are 

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

 stomach of this simple oqj^nism, ai*l the opening of die 

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



