98 
The Irish Naturalist. 
April, 
causes which, on the Darwinian hypothesis, should exercise 
no effect on them whatever. Dr. Weismann adopts the remark- 
able view that as some kinds of anirr»als are much more 
exposed than others, in proportion to their fertility, to the 
attacks of enemies and to other catastrophes, the only way in 
which these animals can maintain their numbers is to develop 
a greater average length of life than is enjo3'ed by the less 
persecuted forms. This would, no doubt, be a most ingenious 
way out of a difficulty, if an animal's foes were not those of its 
own household. But it would, I think, be alike useless and 
uncalled-for, in fact paradoxical, if we accept Darwin's view 
that all' animals are at a numerical high- water mark, and 
suffering constant checks in their efforts to surge over it. 
To quote Professor Weismann's argument in the case of 
birds, he says (accepting the correctness of Darwin's and 
Wallace's view that the number of individuals of each species 
remains tolerably constant from year to year) that if we assume 
that " the individuals of a certain species live for ten years, 
and that they lay twenty eggs in each year," then " of the 200 
eggs which are laid during the ten years which constitute the 
lifetime of an individual, 198 must be destroyed, and only two 
will reach maturity, if the number of individuals in the species 
is to remain constant" So far, there is at least no friction 
between Weismann's and Darwin's points of view ; but from 
this we are requested by Weismann to draw the inference as 
a mathematical certainty, that if the average life of the indi- 
vidual had been only eight years instead often, and only 160 
eggs instead of 200 had been laid, the causes which on the one 
estimate destroyed 198 out of 200 would on the other destroy 
the same proportion out of 160, leaving a surviving residuum 
of less than two ; so that the survivors would be fewer and 
fewer in each succeeding generation, and the species would 
disappear. " It follows," says Professor Weismann, after 
enumerating the many perils to which they are subject, "that 
a relatively long life is the only means by which the mainte- 
nance of the species of birds can be secured." This most 
important inference can only be true on the supposition that 
Darwin was absolutely wrong in his belief that animals are so 
numerous as to have to compete with one another for means 
to live. For in that case it would not matter whether the 
