870 TEXTBOOK OP ZOOLOGY 



did not subscribe, however, to the recapitulation theory when it 

 was formulated later. Haeckel, coming a little later, became con- 

 vinced that the developing embryo lives over again the stages 

 through which its whole race has passed, and he formulated the 

 recapitulation theory or biogenetic law from this idea. In other 

 words, the organism in its individual life tends to recapitulate the 

 different stages through which its ancestors have passed in their 

 racial history. Briefly this same statement is, ontogeny recapitulates 

 phytogeny. The rehearsal of the phylogeny is in rather slurred form 

 in some details, but the basis for the idea is readily seen. In brief, 

 the theory is applied by comparison. Nearly all metazoan organ- 

 isms begin life by the union of the two germ cells to form a single- 

 celled zygote which is the new organism. At this time it is com- 

 parable to the Protozoa. During the ensuing cleavage divisions a 

 colonial form is represented. Following this, when one side infolds 

 to form a gastrula with two germ layers, the embryo is almost 

 identical to the diploblastic coelenterates as represented by Hydra. 

 Following this, the third germ layer forms between the others and 

 results in the triploblastic metazoan. 



Basis for the Theory of Evolution 



One who has thoughtfully studied the field of zoology soon realizes 

 many relationships or homologies in structural make-up of certain 

 different groups of animals ; much similarity of embryonic develop- 

 mental processes, and fundamental coincidences in physiological 

 activities in all living material. Too, it is recognized that fauna 

 and flora are not the same in all parts of the earth at the present, 

 and have not been the same in the past as now. These realizations 

 and other similar ones have been based on scattered sources of evi- 

 dence, much of which is indirect. The evidences which have been 

 discovered, however, have led to the formulation of the theory of 

 evolution and its rather general acceptance as a working hypothesis 

 among biologists. There has been confusion regarding this subject 

 through failure to distinguish between the existence of progressive 

 evolutionary changes, the course of these changes, and the cause of 

 such a progressive series of events. The latter point has been the 

 basis of most of the questions concerning the whole idea, and it is 

 the most speculative of the three. These three are separate though 

 related factors. 



