300 THE CASE AGAINST EVOLUTION 



the growth of young mammals. This took it out of the class 

 of useless vestigial organs, but the recent discovery that it is 

 indispensable to birds as furnishing a secretion necessary for 

 the formation of the tertiary envelopes (egg membrane and 

 shell) of their eggs, has tended to revive the idea of its being 

 a vestigial organ inherited from the lower vertebrates. 



Thus Dr. Oscar Riddle, while admitting that the thymus 

 gland in man has some influence on the growth of the bones, 

 contends that the newly-discovered function of this gland in 

 birds is much more important, since without it none of the 

 vertebrates, excepting mammals, could reproduce their young. 

 "It thus becomes clear," he says, "that though the thymus 

 is almost without use in the human being, it is in fact a sort 

 of 'mother of the race.' The higher animals could not have 

 come into existence without it. For even while our ancestors 

 lived in the water, it was the thymus of these ancestors which 

 made possible the production of the egg-envelopes within which 

 the young were cradled and protected until they were ready 

 for an independent life." (Science, Dec. 28, 1923, Suppl. XIII, 

 XIV.) 



This conclusion, however, is far too hasty. For, even if 

 we disregard as negligible the minor function, that Riddle 

 assigns to the thymus in man, there remains another possi- 

 bility, which H. H. Wilder takes into account, namely, that 

 the thymus may, in certain cases, be a temporary substitute 

 for the lymphatic vessels. Having called attention to certain 

 determinate channels found in some of the lower vertebrates, 

 he tells us that these "can well be utilized as adjuncts of the 

 lymphatic system until their function can be supplied by 

 definite lymphatic vessels." He then resumes his discussion 

 of the lymph nodules in mammals as follows: "Aside from 

 the solitary and aggregated nodules, both of which appear to 

 be centers of origin of lymphocytes, there are numerous other 

 places in which the cellular constituents of the blood are de- 

 veloped. Many of these, as in the case of the aggregated 

 nodules of the intestines, are developed within the wall of the 



