874 



THE AMERICAN NATURALIST 



[Vol. XLV 



ited by the genus, one by one. by the extirpation of the inter- 

 mediate forms. It often happens, also, that changing ecological 

 conditions at the center of distribution of the genus, such as the 

 local development or introduction of predaceous forms, or of 

 external or internal parasites, destroys the typical form there, 

 leaving only aberrant types: or they may even obliterate all 

 traces of the genus. 



Very old genera are thus characterized by having but few- 

 species in widely separated localities, each widely different from 

 the others. These are usually (and rightly) regarded as repre- 

 senting a family composed of a few monotypic, or nearly mono- 

 typic, genera. 



Very often old genera undergo what has aptly been termed an 

 "explosion" of the intergeneric characters, and are then com- 

 posed wholly, or almost wholly, of curious and eccentric species: 

 again a genus in its senescence often is marked by a great devel- 

 opment of certain characters at the expense of others, which 

 usually leads to prompt extinction. In certain localities large 

 numbers of species are remarkable for their eccentric develop- 

 ment, and the exaggeration of certain characters out of all pro- 

 portion to the others, which, so far as we can see, serves no useful 

 purpose. Such localities from a zoological point of view must be 

 considered as old and to have persisted in their present state be- 

 yond the normal life cycle of the genera which have given rise to 

 the erratic species. Just as the life cycle of different animals 

 varies enormously, so does that of species and of genera. Scores 

 of genera belonging to the higher groups of the animal kingdom 

 may arise, grow strong, decline, and finally, with a grand "ex- 

 plosion" of their characters, disappear, before a genus belonging 

 to one of the lower groups, of earlier origin, has reached the 

 summit of its strength. 



In discussing genera, as well as species, one must always keep 

 in mind that for all animals there are two, and for aquatic 

 animals three types of distribution, viz., (1) geographical, with 

 purely inorganic physical barriers; (2) ecological, with wholly 

 organic harriers, consisting of presence or absence of food and 

 predaceous or parasitic enemies; and (3) bathymetric, again 

 with purely physical harriers of pressure and temperature, the 

 latter common lv being the more important with lower animals, 

 the former with the higher. 



Austin Hobart Clark. 



U. 8. National Museum. 



