688 The Outline of Science 



still another form of propagation, known as parthenogenesis, is 

 often to be found. 



Mothers but No Fathers 



Parthenogenesis consists in this, that an egg develops with- 

 out uniting with a sperm. In order to make sure of the fusion 

 of nuclei which is the essential of sexual reproduction, eggs are 

 usually rendered incapable of developing without some stimulus 

 afforded by the sperm's entry. Parthenogenetic eggs need no 

 stimulus and start to develop as soon as mature; so that once 

 more, but in another form, we find reproduction as a special case 

 of unlimited growth. Parthenogenesis is found in such creatures 

 as plant-lice (Aphides) and a good many other insects, water- 

 fleas, and wheel-animalcules, which all reproduce by its means 

 throughout the summer and only produce males in the autumn; 

 and as we shall see later, it can be artificially provoked in many 

 other forms of life. A drone bee, too, develops from an unfer- 

 tilised egg; it has a mother, but no father; whereas the queens 

 and workers arise from fertilised eggs. 



To sum up, we may say that reproduction is always the 

 result of growth, and always must be the separation of one part 

 of an organism from the rest. As life evolves, the part separated, 

 at first equal to the rest of the organism, becomes proportionately 

 smaller and smaller; and the sexual process, at first antagonistic 

 to reproduction, becomes associated with it, at first in part and 

 finally altogether. 



6 

 Regeneration 



The power of unlimited growth is at the bottom not only 

 of ordinary reproduction but of regeneration as well. We are 

 apt to look upon regeneration as something marvellous, because 

 it does not occur to any appreciable extent in ourselves, or in any 

 of the higher animals with which we are familiar ; but, as a matter 



