72 Scientific Proceedings, Royal Dublin Society. 



organism to be ascribed ? It would appear as if these causes, 

 originated in the fact of growth and reproduction, in the adapta- 

 tion of the organism, and in the conditions of limited energy 

 imposed by the environment. 



Under conditions of want, it has been observed, the vegetative 

 division of the organism gave place generally to more numerous 

 sexual reproduction. Active male units are thus sent abroad into 

 the environment. The effect of this is to secure, at the approach 

 of a change in the nutritive level of its environment, its adaptation 

 to the new conditions. For the active and more mobile male units 

 become selectively acted upon by the various contending forces in. 

 the environment. Now it is likely that supposing many genera- 

 tions alive at the one time, a struggle will begin among the 

 members of the species, for the particular form of energy most 

 available to all its members. It is also probable that the younger 

 units, being the more highly adapted to the change of conditions, 

 will get the better of the older members, will in fact destroy them, 

 if in no other way, by starvation. The struggle will, probably, be 

 a lono- one but the older members must finally yield before the in- 

 creasing numbers and adaptation of the younger. Here the 

 effect on the individual is from without, and although traceable to 

 the plasticity of the organism is not inherent in it. 



It will be seen that members of the one species will each 

 successively be assailed by death at a similar stage of development, 

 and at a similar earlier stage will feel the pressure of the keen 

 competition. The period of life when these conditions come about 

 will be in general independent of the rate of supply afforded by the 

 particular environment, but will depend on the rate of reproduction, 

 the period required by the organism to attain to maturity, and the 

 rate at which natural selection is operating to advance the evolution 

 of the species. The action of the last will cause secular variation in 

 the other factors, so that the time-limit, which eventually becomes 

 hereditary, imposed by these factors, will doubtless slowly vary. Con- 

 tending species, so far as they affect the rate of multiplication of the 

 first species by preying upon its younger members, will also enter 

 into the factors which are here crudely summarised as probably 

 determining ultimately the time-limits assigned to the individual. 

 By these factors, too, the events of life-history will be mapped out 

 in time in an approximately constant way. How the effects of these 



