ADAPTATION 211 



mentioned, and the effectiveness of teeth and claws of carnivorous 

 animals for seizing and tearing their prey. Such niceness of adap- 

 tation is universal. Several attempts have been made to explain 

 its attainment; these we shall consider later. 



Caenogenesis, Neoteny, and Paedogenesis. In some organisms 

 metamorphosis enables the individual to occupy different habitats 

 at different periods of its life. The adaptations of earlier stages 

 may be wholly different from those present at maturity and are 

 sometimes even more wonderful examples of fitness for a given 

 environment. The development of such temporary adaptations 

 has been called caenogenesis. In some cases caenogenetic modifica- 

 tions have apparently been of greater benefit to the species than 

 adult adaptations and have been carried over into the adult stage. 

 This condition is known as neoteny. A more extreme emphasis 

 upon the value of caenogenetic adaptations is found in species 

 which attain sexual maturity while still in an immature stage 

 morphologically. This phenomenon is called paedogenesis. 



The development of familiar insect larvae, such as the cater- 

 pillars, maggots, and hellgramites, is caenogenetic. Of these the 

 caterpillar at least is familiar to everyone, and the complete lack 

 of resemblance between it and the adult butterfly or moth into 

 which it develops. Neoteny is illustrated by the axolotl, a sala- 

 mander found in Mexico, which remains an aquatic form and 

 retains its larval gills throughout life. Salamanders usually de- 

 velop into terrestrial adults and this metamorphosis can be arti- 

 ficially induced in the axolotl. Excellent examples of paedo- 

 genesis have been reported in a few species of insects from the more 

 primitive families of two-winged flies. Both larvae and pupae 

 have been observed to produce young in these families. 



The Environment. The environment to which organisms are 

 adapted is complex. We recognize that every organism has an 

 association with the surrounding world, from which it receives 

 stimuli of a chemical and physical nature, as well as the materials 

 of which its body is composed. To this environment it responds 

 by more or less complex reactions. By its intricate responses and 

 by the ultimate return of the substances which it has used during 

 its life, every organism contributes to the complexity of the en- 

 vironment of every other organism. The trees shade other plants, 

 and so modify their relations to sunlight. They also give a home 

 to arboreal animals, and when they die, furnish food for insects, 



