THE EVOLUTION OF ANIMALS. 



391 



the egg to the adult, repeats in an abbreviated 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? Because 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 tad- 

 poles 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. 



388. 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 that are bring- 

 ing about this result and how they work in the process. 

 We doubtless have not found all the factors; those men- 

 tioned below are certainly some of the more important. 



389. Variability and Variation. It is clear that there 

 could be no evolution if organisms could not change or 



