OF WASHINGTON. 125 



and others in its general application to the formation of species.* 

 Natural selection has undoubtedly operated in the past, and is 

 still operating, to fix more or less absolutely the limit of individual 

 insect life in any particular state, according to the necessities of 

 life in a particular species in relation to its environment and to 

 other organisms. But the recognition of this fact does not ex 

 plain why, in two species of the same genus, under like condi 

 tions, the one should hold its own, by producing a single annual 

 generation, equally as well as the other, which produces two or 

 more ; nor does it explain why different species in the same genus 

 or the same family should differ in length of life under like con 

 ditions. For we may justly argue that what was essential in the 

 one case would be essential in the other. It is preferable, it 

 seems to me, to recognize the limitations of natural selection, and 

 to view life, especially insect life, as possessing inherent powers 

 of adaptability and variability. Adaptivity, exhibited by the 

 individual and confirmed or fixed by descent, is powerfully influ 

 enced by natural selection. Variability cooperates with adap- 

 tivity, and gives full scope to natural selection in fixing useful 

 qualities, but it also manifests itself in lines which are not neces 

 sarily essential and in which natural selection plays little or no 

 part ; nay, further, in lines that are purely fortuitous. 



These I believe to be general truths in evolution, and they are 

 just as applicable to the subject of longevity as to the subject of 

 structure and habit ; and it is only by recognizing these limita 

 tions of natural selection that we can get at the true meaning of 

 longevity in animals or of the phenomena of life generally. In 

 short, nature is kaleidescopic, and no single law that we may 

 formulate, however important or however wide its application, 

 will explain all her varied manifestations. No gown can be 

 made " all-sufficient " to fit an unlimited subject. 



The address was discussed by Messrs. Fernow, Schwarz, Ash- 

 mead, and Riley. Mr. Fernow said that a comparison between 

 plants and animals in the matter of longevity can hardly be made, 

 since in plants the functional parts are renewed, while in animals 

 they remain the same. Theoretically, a plant may never die. 

 Weismann's idea of immortality is misleading, as generally 

 stated, since the reproductive cell alone is immortal the individ- 



* Address before the section of Biology, A. A. A. S., Vol. xxxvn, 

 1888, "On the Causes of Variation in Organic Forms;" " Some Inter 

 relations of Plants and Insects," Proc. Biol. Soc., Wash., 1892, vii, pp. 

 81-104. 



