THE GROUND BETWEEN ANIMALS AND PLANTS. 643 



regular motion, was made out by Bonaventura Corti a century ago; 

 but the fact, important as it was, fell into oblivion, and had to be re- 

 discovered by Treviranus in 1807. Robert Brown noted the most 

 complex motions of the protoplasm in the cells of Tradescantia in 

 1831 ; and now such movements of the living substance of plants are 

 well known to be some of the most widely-prevalent phenomena of 

 vegetable life. 



Agardh, and other of the botanists of Cuvier's generation, who 

 occupied themselves with the lower plants, had observed that, under 

 particular circumstances, the contents of the cells of certain water- 

 weeds were set free and moved about with considerable velocity, and 

 with all the appearances of spontaneity, as locomotive bodies, which, 

 from their similarity to animals of simple organization, were called 

 " zoospores." 



Even as late at 1845, however, a botanist of Schleiden's eminence 

 deals very skeptically with these statements ; and his skepticism was 

 the more justified since Ehrenberg, in his elaborate and comprehen- 

 sive work on the Infusoria^ had declared the greater number of what 

 are now recognized as locomotive plants to be animals. 



At the present day, innumerable plants and free plant-cells are 

 known to pass the whole or part of their lives in an actively locomo- 

 tive condition, in no wise distinguishable from that of one of the sim- 

 pler animals ; and, while in this condition, their movements are, to all 

 appearance, as spontaneous as much the product of volition as 

 those of such animals. 



Hence the teleoloirical arojument for Cuvier's first diagnostic char- 

 acter the presence in animals of an alimentary cavity, or internal 

 pocket, in which they can carry about their nutriment, has broken 

 down so far, at least, as his mode of stating it goes. And, with the 

 advance of microscopic anatomy, the universality of the fact itself 

 among animals has ceased to be predicable. Many animals of even 

 complex structure, which live parasitically within others, are wholly 

 devoid of an alimentary cavity. Their food is provided for them, not 

 only ready cooked, but ready digested, and the alimentary canal, be- 

 come superfluous, has disappeared. Again, the males of most rotifers 

 have no digestive apparatus ; as a German naturalist has remarked, 

 they devote themselves entirely to the " Minnedienst," and are to be 

 reckoned among the few realizations of the Byronic ideal of a lover. 

 Finally, amid the lowest forms of animal life, tlie speck of gelatinous 

 protoplasm, which constitutes the whole body, has no permanent di- 

 gestive cavity or mouth, but takes in its food anywhere ; and digests, 

 so to speak, all over its body. 



But, although Cuvier's leading diagnosis of the animal from the 

 plant will not stand a strict test, it remains one of the most constant 

 of the distinctive characters of animals. And, if we substitute for the 

 possession of an alimentary cavity the power of taking solid nutri- 



