wider or sometimes to narrower possibilities. Mollusks took advantage of the path, and many of them have reached even to life arboreal, while some, apparently disheartened or diverted by seductive opportunities, have gone back to water, still retaining, however, their air-breathing organs to prove the roundabout course of their travels.
The ancestral forms of insects doubtless followed the same broad way, but so remote are these ancestors that we may best consider the insects as primitively air-breathing. Some indeed are now aquatic, but still air-breathers, and I doubt not have taken to aquatic life as a fairly modern accomplishment. Vertebrates, however, give us the greatest advance, for from the gilled fish, and early amphibian, to bird or mammal is a long and striking course. If any branch of animals is to be thought of as having had its origin on land rather than in water, it must be the insects, that is, the immediate ancestral form to all the groups of insects. Following back their ancestral line still further we should doubtless reach an aquatic animal, but one not to be recognized as in any degree insect-like in character.
But life in the open air has not confined itself to particular places or conditions. The pressure to occupy each niche of available territory is as strong here as in the water. Here too we may trace certain well-worn paths—paths that have been common to more than one group of animals and traveled independently by each. Let us mention some—subterranean, aquatic, terrestrial, arboreal, aerial. How many forms in hosts of different groups have buried themselves more or less completely in mother earth and there found, or made for themselves, all the necessary conditions for successful life. Earthworms, crustaceans, insects too numerous to mention, mollusks and, among vertebrates, frogs, snakes, lizards, turtles, birds, moles, beavers, gophers, groundhogs, badgers, etc. This for the general trend, many of these, however, taking peculiar and tortuous by-paths to reach the end desired. From terrestrial back to aquatic life has been so frequent a course that we can hardly call the road exceptional. So many insects of different groups have become aquatic in either adult or larval life that it was long held that the insects in general were derived from these aquatic groups. None, I think it safe to say, has had such an immediate ancestry, while aquatic beetles, bugs, caterpillars and even dragonflies, caddiceflies, etc., have, I firmly believe, gradually assumed aquatic habits as descendants of forms that lived on land.
We can easily believe that frogs advanced from an aquatic to a terrestrial condition, because we can actually see the process in the individual history of each, but there is reason to believe that even among certain batrachians aquatic life becomes more habitual and succeeds a terrestrial stage. Even the frog himself becomes decidedly aquatic in his old age, and for reptiles, while we often think of them as aquatic,
