distinct enough to be recognized by the classifier, the facts of environment on which they depend should be distinct enough to be discovered by the observer of animal habits.
This proposition can be established only by the connection of structure with habits and with conditions of environment in a large number of groups of different values in each of this types. Its reasonableness is best shown by the fact that it is recognized as true in those groups whose habits and structure are best known. The difficulty of establishing it as a general truth lies rather in the lack of knowledge of animal habits and surroundings than in a lack of knowledge of structure.
A few undoubted examples of adaptation of groups to special environment are of birds to aërial habitat, of fishes to water, of rodents to hard foods, and of squirrels to arboreal seeds with hard coverings.
One of the difficulties in tracing this connection between existing groups and the environment to which they are adapted is in this, that the more fundamental structural characteristics may remain after the animal possessing them has, by later superficial modifications, become adapted to other and perhaps antagonistic conditions, and even after the conditions leading to such structures have disappeared. Of the first case we have such examples. as the ostrich and penguin, which, while retaining their bird characters, have lost flight, and have become, one of them terrestrial and the other aquatic; and the bats and whales, mammals which are no longer capable of existing in the normal mammalian habitat.
Of examples of the last case, animals existing after the conditions leading to their existence have disappeared, we can not be so sure; but the marsupials and the proboscidians among mammals, and the turtles among reptiles, may be examples in point. If animals may become superficially modified so that they may exist under conditions different from those for which they were primarily fitted, they might still exist after such primary conditions had ceased to exist. Whales might exist if all land were destroyed.
Another difficulty in connecting animal forms with special conditions of environment is in the multitude and variety of modifications that have taken place. No type of animal life has stopped at one set of changes. If there was one species of bird, and that fitted in a general way for all bird-life, one of mammals, one of fishes, etc., the problem would be simple; but as soon as a group of animals has become adapted to a new fact of environment it falls under the influence of a new set of surroundings, more or less numerous, from the very fact of such change, and again becomes broken up into subdivisions adapted to each of
