Among the Crustacea, two orders, the Phyllo carida and the Synearida, have been recognized by Packard. and the latter group also recognized by Oilman, as being generalized forms which have given rise to the higher more specialized groups represented by the modern shrimps, lobster, and crabs.
Perhaps the most important paleontolo5ical discovery in the last half century, as regards arthropod animals, is that of the appendages of trilobites, which we owe to Walcott, Alatthew. and especially to Beecher. Finally,- the view has of late years been rendered clear that nearly every class of invertebrate animals originated in Precambrian times; the vertebrates with limbs and lungs. Amphibians tl n (1 reptiles. appearing near the close of the Paleozoic Age, while birds and mammals diverge from some reptilian stock in the early Mesozoic.
(4) The fourth period is that of evolution and bionomies. The history of evolution (q.v.) is now a twice-told tale. llow much we are indebted to Darwin (q.v.). Wallaee (q.v.), Herbert Spencer (q.v.), and others has been re counted elsewhere. It remains only to point out smile salient features in the history of the theory of descent a ml its modifications during the nineteenth century. A noticeable eiremn stance in the history of the rise and spread of the doctrine is the way in which the once almost forgotten or misunderstood views of Lamarck (q.v.) have been either unearthed or hit upon independently by others, as Cope, Hyatt, Jack son, Eimer, etc., proving that the Lamarckian factors of change of environment, use and disuse, even if we throw out use-inheritanee as applied to Mutilations, etc., are the primary agencies in modifying organisms and in bringing about not only varieties and species, but also families, orders, classes, and even phyla.
Natural selection also has its foundation in the struggle for existence and competition. And here it is interesting to observe that Darwin and Wallace, who independently thought out the selection idea, both acknowledge their imlebt ednes, to Malthus's "Essay on Population" (1798) for the idea which dominates their theory. It is not, perhaps, generally known that Malthus on his part freely acknowledged in the open ing pages of his essay his indebtedness to Benja min Franklin for the motif of his work. In 1755.
in his Observations Concerning the Increase of Mankind and the Peopling of ('ountries, Franklin enunciated the principle of struggle for existence, and the evil effects of overcrowding if any single species were allowed to multiply unchecked by competition with other forms. "There is, in short," he says, "no bound to the prolific nature of plants and animals. hut what is niade by their crowding and interfering with each other', means of subsistence. Were the face of the earth vacant of other plants, it might be gradually sowed and overspread with one kind only, as. for instance, with fennel, and were it empty of other inhabitants, it might in a few ages be replenished from one nation only, as, for instance, with Englishmen." (Sparks's edition of the Works of B. Franklin, p. 319.) The theory of descent has aroused atten tion and given a decided impulse to the study of bionomies, ethology. or the habits and instincts of animals and their relations to the world around them. The pioneers in this captivating phase of biology were Huber, Reaumur, Reimarus, Sie bold, and Leuckart. and, to come down to later times, Lubbock, Romanes, Wyman. Plateau. Fort, Pabre, Moggridge, Cook, (I. W. and E. G. Peckham. Loeb, Weismann, and Wheeler deserve special mention.
One great difference between plants and ani mals is that the latter move from one place to andther; and how animals orientate themselves and the means or sense-organs by which they move in reference to surrounding objects or organisms, find their food, or their mates, has engaged and is now engaging much attention. Here might be cited recent work on the otoeysts of the inverte brates, and the views now held as to the origin of the vertebrate ear and the lateral sense-organs of fishes and salamanders from organs of orienta tion in the worms. The mechanics of locomotion worked out by the older anatomists and phi -sio logists have, by the ingenious inventions and use of photography, been greatly extended by Marcy.