I.] THE VARIATION IN LONGEVITY, 45 



opportunity was left for new ones; and so the ascent 

 must continue so long as physical conditions which are 

 not absolutely prohibitive of life shall become unlike. 



Species have acquired different degrees of longevity, 

 the same as they have acquired different sizes and shapes 

 and habits, — by adaptation to their conditions of life. 

 Annual plants comprise about half of the vegetable king- 

 dom, and these are probably all specializations of com- 

 paratively late time. Probably the greater part of them 

 were originally adaptations to shortening periods of 

 growth, — that is, to seasonal changes. The gardener, by 

 forceful cultivation and by transferring plants towards 

 the poles, is able to make annuals of perennials. Now, 

 a true annual is a plant which normally ripens its seeds 

 and dies before the coming of frost. Many of our gar- 

 den plants are annuals only because they are killed by 

 frost. They naturally have a longer season than our 

 climate will admit, and some of them are true perennials 

 in their native homes. These plants are, with us, plur- 

 annuals, and amongst them are the tomato, red pepper, 

 egg-plant, potato, castor bean, cotton, lima bean, and 

 many others. But there are some varieties of potatoes 

 and other plants which have now developed into true an- 

 nuals, normally completing their entire growth before 

 the approach of frost. It is all the result of adaptation 

 to climate, and essentially the same phenomenon is the 

 development of the annual and biennial flora of the earth 

 from the perennial. An interesting example of the effect 

 of climate upon the seasonal duration of plants is the 

 indeterminate or prolonged growth of plants in England 

 as compared with the same plants in America. The 

 cooler summer and very gradual approach of winter in 

 England develop a late and indefinite maturity of the 



