22 HISTORY OF THE HUMAN BODY 



the act of birth or hatching. These latter are purely external 

 phenomena and mark no important stage in the development 

 of the animal save in the line of certain necessary adaptations. 

 The birth period often varies considerably in allied forms. 



These external phenomena are wholly adaptive and are 

 regulated by the conditions imposed by the struggle for exist- 

 ence. Thus aquatic salamanders lay eggs which pass through 

 all the stages from the beginning outside of the body of the 

 parent, but in the more terrestrial species, although closely 

 allied to the foregoing, the eggs are detained in the oviducts 

 of the mother, where development continues throughout the 

 larval period and the young are produced in a practically adult 

 condition. 



It is advantageous to some species to produce a large num- 

 ber of immature offspring, relying upon chance for the sur- 

 vival of a few of them ; under other circumstances it has been 

 proven the better course to produce a small number of well- 

 developed young, furnished with a better equipment for fight- 

 ing the battle of life. 



II. In developmental history a given species reproduces in 

 miniature its own ancestral history, and thus passes through 

 those stages only through which its actual ancestors have also 

 passed. 



Thus, in the diagram, form i8& has passed through the 

 stages i8a, 1 8, 17, 6, 5, 4, 3, 2, and I as well as the innumer- 

 able stages between these points as represented by the lines 

 connecting them, but would not reproduce any stage in the 

 history of some allied form through which the latter has 

 passed since the divergence, such as 7 or 19. 



The only stages common to any two recent forms, allied or 

 not, are those below the point represented by their latest com- 

 mon ancestor. This may be formulated as follows : 



III. In any two given forms only those developmental 

 stages which represent common ancestors are the same in 

 both. From the point at which their ancestors diverged their 

 developmental histories are distinct and different. It follows 

 from this that the more closely allied the two forms, the more 



