HEREDITARY TRANSMISSION AND VARIATION. 83 



a very considerable part of their existence, multiply 

 themselves by means of a kind of internal budding, the 

 buds being developed into essentially asexual animals, 

 which are neither male nor female ; they become con- 

 verted 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 suc- 

 cessions ; 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 detach- 

 ment 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 nowerless. 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 themselves, are unable 

 to assume the parental form ; but if they be brought 

 into contact with one another, the effect of the mixture 

 of organic substances proceeding from two sources ap- 

 pears to confer an altogether new vigour to the mix- 

 ed product. This process is brought about, as we all 

 know, by the sexual intercourse of the two sexes, and 

 is called the act of impregnation. The result of this 

 act on the part of the male and female is, that the 

 formation of a new being is set up in the ovule or egg ; 

 this ovule or egg soon begins to be divided and sub- 

 divided, and to be fashioned into various complex or- 



