HEREDITARY TRANSMISSION AND VARIATION. 85 



asexual multiplication, and there are other instances, 

 and still more extraordinary ones, in which this pro- 

 cess takes place naturally, in a more hidden, a more 

 recondite kind of way. You are all of you familiar 

 with that little green insect, the Aphis or blight, 

 as it is called. These little animals, during a very 

 considerable part of their existence, multiply them- 

 selves by means of a kind of internal budding, the 

 buds being developed into essentially asexual animals, 

 which are neither male nor female ; they become 

 converted into young Aphides, which repeat the 

 process, and their offspring after them, and so on 

 again; you may go on for nine or ten, or even 

 twenty or more successions ; and there is no very good 

 reason to say how soon it might terminate, or how long 

 it might not go on if the proper conditions of warmth 

 and nourishment were kept up. 



Sexual reproduction is quite a distinct matter. Here, 

 in all these cases, what is required is the detachment 

 of two portions of the parental organisms, which 

 portions we know as the egg or the spermatozoon. 

 In plants it is the ovule and the pollen-grain, as in 

 the flowering plants, or the ovule and the antherozooid, 

 as in the flowerless. Among all forms of animal life, 

 the spermatozoa proceed from the male sex, and the 

 egg is the product of the female. Now, what is 

 remarkable about this mode of reproduction is this, 

 that the egg by itself, or the spermatozoa by them- 

 selves, are unable to assume the parental form ; but 

 if they be brought into contact with one another, 

 the effect of the mixtare of organic substances pro- 



