such families as the beaver, in such genera as the 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 primarily 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 conditions 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 becomes 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
