416 EVIDENCES OF EVOLUTION 



known to have been so in many instances. As the species 

 evolved, the organ became less important, dwindled in 

 size, and ultimately ceased to function. 



Evidence from Embryology. — In a former chapter 

 (XXVI) it has been shown that all of the many-celled 

 animals develop from a single fertilized egg-cell whose 

 divisions result in a blastula and still more complicated 

 gastrula. This development is from a comparatively 

 simple to a complex condition. This is also the history 

 of life on the earth. The ancient simple species have 

 evolved into the more or less complex descendants of 

 modern times. Amoeba, Vol vox, and Hydra, respec- 

 tively correspond to increasingly complex steps in the 

 evolution of the animal kingdom. The corresponding 

 stages — egg, blastula, and gastrula — are increasingly 

 complex states in the evolution or development of an 

 individual. The young stages in the growth of a higher 

 species thus resemble the adults of lower species. Speak- 

 ing broadly, life history parallels or repeats race history. 



Development of Vertebrates. — This parallelism be- 

 tween life and race histories is further illustrated in a 

 striking manner by the development of the vertebrates. 

 The fishes, frogs, reptiles, birds and mammals all belong 

 to this great group, and agree in the possession of a back- 

 bone, four limbs, a dorsal nervous system, and several 

 other important features. The resemblances are pre- 

 sumably due to the fact that they are the descendants of 

 a common ancestor; the differences are owing to evolu- 

 tionary changes, fitting each species for a definite mode 

 of life. 



In its development each species travels essentially the 

 same path, and for a long period the young can be dis- 

 tinguished only by an expert. Moreover, before definite 

 classes assume the final adult and distinguishing charac- 

 ters they all develop gills in the throat region. Internally 

 as well as externally, they bear a striking resemblance 

 to fishes. In the fishes these organs persist throughout 



