272 THE RHYTHM OF MOTION. 



unceasing conflict between the tendency of a species to in- 

 crease and the antagonistic tendencies, there is never an 

 equilibrium : one always predominates. In the case even of 

 a cultivated plant or domesticated animal, where artificial 

 means are used to maintain the supply at a uniform level, 

 we still see that oscillations of abundance and scarcity can- 

 not be avoided. And among the creatures uncared for by 

 man, such oscillations are usually more marked. After a 

 race of organisms has been greatly thinned by enemies 

 or lack of food, its surviving members become more favour- 

 ably circumstanced than usual. During the decline in 

 their numbers their food has grown relatively more abun- 

 dant; while their enemies have diminished from want of 

 prey. The conditions thus remain for some time favourable 

 to their increase; and they multiply rapidly. By and by 

 their food is rendered relatively scarce, at the same time that 

 their enemies have become more numerous ; and the destroy- 

 ing influences being thus in excess, their number begins to 

 diminish again. Yet one more rhythm, extremely 



slow in its action, may be traced in the phenomena of Life, 

 contemplated under their most general aspect. The re- 

 searches of palaeontologists show that there have been going 

 on, during the vast period of which our sedimentary rocks 

 bear record, successive changes of organic forms. Species 

 have appeared, become abundant, and then disappeared. 

 Genera, at first constituted of but few species, have for a 

 time gone on growing more multiform; and then have 

 begun to decline in the number of their subdivisions : leav- 

 ing at last but one or two representatives, or none at all. 

 During longer epochs whole orders have thus arisen, cul- 

 minated, and dwindled away. And even those wider divis- 

 ions containing many orders have similarly undergone a 

 gradual rise, a high tide, and a long-continued ebb. The 

 stalked Crinoidea, for example, which, during the carbon- 

 iferous epoch, became abundant, have almost disappeared: 

 only a single species being extant. Once a large family of 



