LECTUKE Y. 



THE CONDITIONS OF EXISTENCE AS AFFECTING THE 

 PERPETUATION OF LIVING BEINGS. 



In the last Lecture I endeavoured to prove to you 

 that, while, as a general rule, organic beings tend to 

 reproduce their kind, there is in them, also, a constantly 

 recurring tendency to vary — to vary to a greater or to 

 a less extent. Such a variety, I pointed out to you, 

 might arise from causes which we do not understand ; 

 we therefore called it spontaneous ; and it might come 

 into existence as a definite and marked thing, without 

 any gradations between itself and the form which pre- 

 ceded it. I further pointed out, that such a variety 

 having once arisen, might be perpetuated to some ex- 

 tent, and indeed to a very marked extent, without any 

 direct interference, or without any exercise of that pro- 

 cess which we called selection. And then I stated 

 further, that by such selection, when exercised artifi- 

 cially — if you took care to breed only from those forms 

 which presented the same peculiarities of any variety 

 which had arisen in this manner — the variation might 

 be perpetuated, as far as we can see, indefinitely. 



The next question, and it is an important one for 

 us, is this : Is there any limit to the amount of varia- 



