244 ^^^ POPULAR SCIENCE MONTHLY. 



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 surround- 

 ings than in a lack of knowledge of structure. 



A few undoubted examples of adaptation of groups to special 

 environment are of birds to aerial 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 ex- 

 isting 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 struct- 

 ures 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 terres- 

 trial and the other aquatic ; and the bats and Avhales, mammals 

 which are no longer capable of existing in the normal mamma- 

 lian habitat. 



Of examples of the last case, animals existing after the con- 

 ditions 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 environ- 

 ment 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 



