2 Cunningham, Unisexual Inheritance. 



the powers of development in the germ cells are entirely independent 

 of the body, that germ cells derive all their properties from the germ 

 cells of previous generations. Thus a hen's egg develops into a chick, 

 not because it is produced by a hen but because it is descended from 

 the eggs of previous generations which also had the power of deve- 

 loping into chicks. When one egg develops into a chick, it divides 

 into numerous cells, some of which form the body of the bird, while 

 others are the germs of new eggs, and the body of the bird has no 

 influence on these new germs. The developmental powers of the germ- 

 cells themselves however, as they multiply by subdivision, and unite 

 in sexual union, undergo variations, and therefore the individuals deve- 

 loped from them are not exactly alike for all time, but show, as we 

 observe, individual peculiarities. By constant selection from numerous 

 individuals with small or great peculiarities evolution is supposed to 

 be effected. 



The body of the individual, as distinguished from the germ cells 

 in its .reproductive organs, may be modified by accident or exercise or 

 stimulation. A muscle grows larger when exercised, sunlight causes 

 the skin to become pigmented, the friction of a boot may produce a 

 corn. But such physiological changes, according to We is man n, begin 

 and end with the individual, with the body or soma. We can thus 

 definitely distinguish between variations which arise in the soma, soma- 

 togenic variations, and variations which have their origin in the germ- 

 cell, blastogenic, and according to Weismann somatogenic variations 

 have no effect on the germ cells and therefore never become here- 

 ditary. 



That I am justified in attributing the disbelief in the inheritance 

 of somatogenic variations to theoretical prejudice is proved I think by 

 a passage which Herbert Spencer quotes from a letter addressed 

 to him by a zoological expert at Cambridge. The passage contains 

 this statement: ,,Most of us here at Cambridge are intensely opposed 

 to the doctrine of the inheritability of acquired variations. Even 

 assuming that the developmental power of a germ is determined by 

 its molecular structure, 'we still fail to conceive any means by which 

 for instance a change in the development of a muscle or nerve can 

 effect a corresponding change in that part of the germ which is destined 

 to produce a corresponding part in the descendant." 



There is however another way of testing the rival theories of 

 heredity, besides the possibility of conceiving the mechanism of the 

 process, and that is by comparing the necessary logical consequences 

 of the theories with observed and admitted facts. The theory whose 

 deductions agree more closely with the facts of observation is likely 

 to be the nearer the truth. I have lately devoted considerable labour 

 to making such a comparison for the facts concerning secondary sexual 



