202 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Metaplasis, and Cataplasis. For the processes which 

 bring about the first and last of these conditions, Pro- 

 fessor Hyatt has used the terms Anagenesis and Cata- 

 genesis. Catagenesis is equivalent to degeneracy and 

 has played an important part in organic evolution. I 

 had used the term previously to Professor Hyatt for 

 the same process, but with a wider application ; ex- 

 tending its use to inorganic nature as well.^ (See 

 Chapter IV. of this book.) 



Embryology has, however, revealed another series 

 of phenomena which in many instances obscure the 

 simplicity of the problem of ontogeny as presented in 

 the preceding pages. It was the merit of Haeckel to 

 generalize from the facts brought to light by this sci- 

 ence, so as to present the relations which subsist be- 

 tween the primitive stages of all multicellular animals. 

 This is known as the Gastraea theory. He showed 

 that the primitive gastric cavity of all such animals is 

 produced by an invagination of a portion of the sur- 

 face of a primitive sphere or morula, which results 

 from the segmentation of the oosperm. This hollow 

 half-sphere he termed the gastrula, and the theoretical 

 primitive animal which corresponds to it he called the 

 Gastraea. Marine animals very similar to this Gas- 

 traea have been discovered. Haeckel showed, how- 

 ever, that gastrulas are not all alike, since they differ 

 in the extent to which the segmentation of the oosperm 

 may be carried, and the rate of segmentation of differ- 

 ent parts of it. Thus early do inexact parallelisms 

 arise. From this point onwards special peculiarities 

 of the various developmental lines appear, some of 

 which have especial reference to the necessities of em- 

 bryonic life. Hence the trochosphere stage of so many 



lOrigzn of ike Fittest, p. 422. 



