66 PERPETUATION OF LIVING BEINGS 



hidden, a more recondite kind of way. You are all of you 

 familiar uith those little green insects, the Aphis or blight, 

 ns it is called. These little animals, during a very con- 

 siderable 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 converted into 

 young Aphides, which repeat the process, and their off- 

 spring 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 and 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 themselves, 

 arc 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 

 appears to confer an altogether new vigour to the mixed 

 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 

 [ of the male and female is, that the formation of a new 

 ag is set iij) in Hie ovule or egg ; this ovule or egg soon 

 ins to be divided and subdivided, and to be fashioned 

 complex organisms, and eventually to develop 

 into the form of one of its parents, as I explained in the 

 first lecture. These are the processes by which the per- 

 uation of organic brings is secured. Why there should 

 I he two modes why this rc-invigoration should be 

 required on the part of I he female element we do not know ; 

 bill arcdiy (he fael, and it is presumable, that, 



however long the process of asexual multiplication could 

 be continued, 1 say there is good reason to believe that 

 it would come to an end if a new commencement were not 

 obtained by a conjunction of the two sexual elements. 



