140 How Living Things Come to be What They Are 



urchins with the successive stages in the development of 

 living species, revealing a remarkable parallelism. 



Many such parallelisms between the embryonic stages 

 of late forms and the succession of earlier fossil forms have 

 been pointed out. One particularly interesting example is 

 presented by the fossil series of ammonites, animals resem- 

 bling the nautilus, and young stages of later forms also found 

 in the rocks. 



Divergence in Human Development 



We accept the human race as one " species," taking for 

 granted a common origin, whatever we may think of evo- 

 lution. The facts of human development, when comparisons 

 are made among the various races, show as we should expect 

 a consistent parallelism from stage to stage, notwithstanding 

 the marked differences which we see among the adults of 

 these races. More intensive recent studies show further that 

 the earlier in its development an embryo is examined, the more 

 does it resemble other human embryos; the older the em- 

 bryt), the more distinct is it, the more individualized. More- 

 over, where careful measurements have been made it is found 

 that the embryos of different races diverge more and more 

 from each other as they advance in development. The em- 

 bryos of a given race will progressively differ more from 

 embryos of another race, as they grow older. Never is there 

 any evidence of convergence, or of becoming more alike. 



Careful measurements of the various structures in hu- 

 man and other primitive fetuses show that as growth proceeds 

 the shape of the head, the proportions of the limbs and other 

 features become increasingly different, as between one species 

 and another. For a period the ratio of one organ's dimen- 

 sion in two forms may remain the same — but the differences 

 noted at one point never become less. Whether it be the 

 feet or the hands, the jaws or the brain box, whatever parts 

 are compared in the embryos of two different species of 

 primates become progressively more different from each 

 other as development goes forward. 



