III.] LIFE AND DEATH. 1 57 



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 increas- 

 ingly 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 hfe 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 development take place under the shelter of the 

 parent organism. Associated with such a process there is 

 frequently a necessity for the germs to reach a certain spot, 

 where alone their further development can take place. Thus a 

 segment of a tapeworm lives until it has brought the embrj^os 

 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 offspring 

 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 depo- 

 sition of eggs at the appropriate time, place, etc., do not survive 

 this act ; and the duration 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. 



But the lengthening of the reproductive period alone may 



