ANIMALS AS MODIFIED BY ENVIRONMENT. 247 



sucli families as the beaver, in sucL. genera as tlie otter and 

 muskrat and flying-squirrel, and in such species as the water- 

 hare of the Southern States. 



In addition to the class of conditions above mentioned which 

 have led to fundamental differences of breathing organs and of 

 organs of locomotion especially, there are others which have had 

 a much more superficial effect in modifying life. These are such 

 as foods, enemies, climate, and perhaps others too subtile to be 

 known at present. These differ from the first in having come 

 into existence, many of them, since the creation of life, and in 

 having been in a continual state of change. The groups called 

 orders, families, genera, and species, depend on these in most 

 cases for their reason for existence. The structures modified lead 

 to differences of size, color, shape, teeth, or other organs of food- 

 taking, and secondary modifications of the organs of locomotion. 

 Like the first class of conditions mentioned, these may also by 

 their continual existence cause modifications in other groups 

 than those especially and i^rimarily fitted to them. The primates 

 are fruit-eaters and the bats are normally insect-eaters ; but the 

 fruit-bats are secondarily modified in teeth, size, stomach, etc., 

 for fruit-eating. 



The groups founded on these secondary and changing con- 

 ditions have also been in a corresponding state of change, old 

 forms disappearing and new ones taking their place. Where 

 facts of this second class have approached stability, the groups 

 corresponding have partaken of this character in the same 

 degree. The foods which became such important factors in the 

 modification of mammals must have at a very early period taken 

 on the general characters of fruits, flesh, insects, grass, and hard 

 substances, and the great orders were at an early period formed 

 and have remained, and must do so while the earth exists in 

 anything like its present state. 



Many of the so-cailed families are also based upon conditions 

 which have a good degree of permanence ; but as the lesser groups 

 are approached the facts of environment upon which they are 

 established become more and more narrow and more capable of 

 either destruction or change. While the general class of hard 

 foods may remain as long as terrestrial life exists, particular 

 kinds of nut-trees or of grains may disappear, and with them 

 species and even genera depending upon them for existence. As 

 the conditions leading to the formation of the lesser groups grow 

 more narrow in their character, being limited perhaps by a single 

 species of food, the location in which this condition exists be- 

 comes restricted also, and so the chances for its destruction are 

 increased. But very many of the changes among the ultimate 

 groups are not by destruction, but by change of the conditioning 



