492 ZOOLOGY 



individual animal, in passing from the egg to the adult, repeats 

 in an iabbreviated way, in a few days or years, many of the steps 

 taken by the race to which it belongs, in its evolution from its 

 single-celled ancestors to its present condition." Put briefly, it 

 reads : Individual history is a brief recapitulation of race history. 



Why can the individual egg cell of a frog, in a few months, 

 pass through a morula, a blastula, a water-breathing legless fish- 

 like stage, and into an air-breathing, four-legged adult? Be- 

 cause its ancestors were first single-celled animals, and through 

 millions of years and countless generations gradually developed 

 first into a mass of cells, later into a gastrula-like stage, and later 

 still for long ages breathed by gills, like the fishes, and last 

 of all became what they now are — water-breathing tadpoles 

 with the power to breathe air in the adult life. 



Because the history is so much shortened, only some of the 

 more profound stages, are recapitulated, and the advances of the 

 race in thousands of years are compressed into moments in the 

 individual history. Furthermore, each species has introduced 

 idiosyncrasies that belong in no way to the race; For these 

 reasons the general truth of recapitulation must not be applied 

 too literally or widely. 



489. The Principal Factors Entering into Evolution. — ^As was 



indicated earlier, biologists no longer question the fact of the 

 evolution of the animal kingdom. They are now seeking to 

 find the principal elements (causes) that are bringing about this 

 result and the manner in which each contributes to the process. 

 We doubtless have not found all the factors. However, those 

 mentioned below certainly enter into the explanation. 



Evolution could not take place without variation, nor with- 

 out some device to preserve {i.e., repeat) and to accumulate these 

 variations from generation to generation after they arise. Fur- 

 thermore, while variation may be in all directions, evolution 

 seems to follow rather definite courses. These courses are not 

 haphazard. They have on the whole led toward adaptation of 

 the organisms to their most pressing surroundings. In other 

 words there . seems to have been selection and elimination of 

 variations, and thus a guidance of evolution. 



