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 preceded it. I further pointed out, 

 that such a variety having once arisen, might be per- 

 petuated to some extent, and indeed to a very marked 

 extent, without any direct interference, or without any 

 exercise of that process which we called selection. And 

 then I stated further, that by such selection, when exercised 

 artificially 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 variation from 

 the primitive stock which can be produced by this process 

 of selective breeding ? In considering this question, it 

 will be useful to class the characteristics, in respect of 

 which organic beings vary, under two heads : we may 

 consider structural characteristics, and we may consider 

 physiological characteristics. 



In the first place, as regards structural characteristics, 

 I endeavoured to show you, by the skeletons which I had 

 upon the table, and by reference to a great many well- 

 ascertained facts, that the different breeds of Pigeons, the 

 Carriers, Pouters, and Tumblers, might vary in any of 

 their internal and important structural characters to a 

 very great degree ; not only might there be changes in the 

 proportions of the skull, and the characters of the feet 



