490 THE AMERICAN NATURALIST [Vol. XLII 



tendency to smallness of size especially in those species 

 having comparatively little vitality to start with. It 

 could happen then that from this cause alone either all or 

 but a few of the species of a fauna would become dwarfed. 



The hard parts of the animal are so intimately related 

 to the soft parts that whatever affects the soft parts is 

 immediately impressed upon the building shell. Thus 

 whatever ill or good conditions the animal is subjected to 

 are expressed in the shell, as are moist and dry summers 

 recorded by the annual rings of exogenous trees. 



Two classes of dwarf faunas are noted: (1) Faunas 

 where the individuals are of smaller size than that to 

 which the species grows under normal conditions; this is 

 the resultant of an abnormal habitat. (2) Faunas where 

 all the individuals are small but of the normal size of the 

 species; in this case some selective action has weeded 

 out all the large and heavy species, leaving a dwarf but 

 not stunted fauna. Dwarf faunas usually include repre- 

 sentatives of both these classes. 



Dwarfing shows itself in two ways : 



1. An acquirement of old age characters by the dwarfed 

 animal. Such old age characters as sluggishness, loss of 

 external sculpture, teeth, etc., are verv common in stunted 

 forms. 



2. A retention of youthful characters. That is, the 

 animal's growth takes place so slowly, due to the ab- 

 normal environment that saps the vitality, that death 

 overtakes it before it has passed through the youthful 

 stage, and the animal develops no old age characters. 



