LIFE AND DEATH. 155 



ber of the higher animals. But such an association, tog-ether with 

 the simultaneous ripening- of the reproductive cells, has not been 

 maintained continuously in the past. As the soma becomes larger 

 and more highly organized, it is able to withstand more injuries, 

 and its average duration of life will extend : pari passu with these 

 changes it will become increasingly advantageous not only for the 

 number of reproductive cells to be multiplied, but also for the time 

 during which they are produced to be prolonged. In this manner 

 a lengthening of the reproductive period arises, at first continuously 

 and then periodically. It is beyond my present purpose to consider 

 in detail the conditions upon which this lengthening depends, but 

 I would emphasize the fact that a lengthening of life is connected 

 with the increase in the duration of reproduction, while on the 

 other hand there is no reason to expect life to be prolonged 

 beyond the reproductive period ; so that the end of this period is 

 usually more or less coincident with death. 



A further prolongation of life could only take place when the 

 parent begins to undertake the duty of rearing the young. The 

 most primitive form of this is found among those animals, which 

 do not expel their reproductive cells as soon as they are ripe but 

 retain them within their bodies, so that the early stages of develop- 

 ment take place under the shelter of the parent organism. Associ- 

 ated with such a process there is frequently a necessity for the 

 germs to reach a certain spot, w r here alone their further development 

 can take place. Thus a segment of a tapeworm lives until it 

 has brought the embryos into a position which affords the possibility 

 of their passive transference to the stomach of their special host. 

 But the duration of life is first materially lengthened when the off- 

 spring begin to be really tended, and as a general rule the increase 

 in length is exactly proportional to the time which is demanded by 

 the care of the young. Accurately conducted observations are 

 wanting upon this precise point, but the general tendency of the 

 facts, as a whole, cannot be doubted. Those insects of which the 

 care for their offspring terminates with the deposition of eggs at the 

 appropriate time, place, etc., do not survive this act ; and the dura- 

 tion of life in such imagos is shorter or longer according as the 

 eggs are laid simultaneously or ripen gradually. On the other hand, 

 insects such as bees and ants which tend their young, have a life 

 which is prolonged for years. 



