392 THE CAUSES OF THE XI 
I now, then, take up the next question, What 
do we know of the reproduction, the perpetuation, 
and the modifications of the forms of living beings, 
supposing that we have put the question as to their 
origination on one side, and have assumed that at 
present the causes of their origination are beyond 
us, and that we know nothing about them? Upon 
this question the state of our knowledge is ex- 
tremely different ; it is exceedingly large: and, if 
not complete, our experience is certainly most ex- 
tensive. It would be impossible to lay it all before 
you, and the most I can do, or need do to-night, is 
to take up the principal points and put them be- 
fore you with such prominence as may subserve 
the purposes of our present argument. 
The method of the perpetuation of organic beings 
isof two kinds,—the non-sexual and the sexual. In 
the first the perpetuation takes place from and by 
a particular act of an individual organism, which 
sometimes may not be classed as belonging to any 
sex at all. In the second case, it is in con- 
sequence of the mutual action and interaction of 
certain portions of the organisms of usually two 
distinct individuals,—the male and the female. The 
cases of non-sexual perpetuation are by no means 
so common as the cases of sexual perpetuation ; 
and they are by no means so common in the animal 
as in the vegetable world. You are all probably 
familiar with the fact, as a matter of experience, 
that you can propagate plants by means of what 
