X. THE MECHANISM OF EVOLUTION 413 



however, this salamander may develop into the adult form of lung- 

 breathing. The administration of thyroid gland substance seems most 

 effective to lead the salamander to the adult form, suggesting that the 

 inability to recover the adult form may be involved in the deficiency 

 of thyroid hormone. 



Environmental effects, as previously stated, appear to exert their 

 influences upon genes mainly through hormones. Accordingly, hormones 

 only, without accompanying any environmental effects, may naturally 

 be able to induce certain changes in genes. As discussed previously, 

 hormones play an important role in the establishment of individual 

 development. The metamorphosis of ^.tadpoles is accelerated by the 

 administration of thyroid-gland substance and also, as is well known, 

 that of insects is subjected to the function of endocrine glands. The 

 pattern of individual development of the stag antler may show the 

 pattern of its phylogenical development, but its development is inter- 

 rupted by the castration, showing also the involvement of hormones. 

 Thus certain environmental alterations having influences upon hormones 

 may cause atavism as in axolotl. 



The primary organisms were generated and evolved in water, so 

 that their parting from water might be achieved only with unusual 

 difficulties. In like manner, for the secondary organisms the departure 

 from the parasitism may also be not an easy task. However, the re- 

 turn of land animals to water or of free-living secondary organisms 

 to parasitism may occur comparatively easily, because the return must 

 only be atavism or the recovery of the former structure. To be para- 

 sitic at the larval stage and free-living at the adult may be a natural 

 form of the individual development recapitulating the phylogeny. The 

 reverse is, however, unnatural, and so may be a type of atavism. 



The advancement of parasitic animals into free living may be thus 

 possible. On the other hand, the entry of free-living into parasitism 

 appears to be impossible, because any spontaneous production of char- 

 acters which are suited to parasitism, a most particular and extremely 

 restricted environment, cannot be considered. From this point of view, 

 the writer regards animals which are parasitic at any stage of life as 

 the secondary organisms. 



