The Internal Causes are: 



(1) Cell Division. — As growth is the result of cell divi- 



sion, and as all cell divisions involve very in- 

 tricate divisions and re-arrangements of the 

 chromosomes of the nucleus (see chapter 16) 

 ^ and the smaller determinants or physiological 

 units, it would be strange if differences 

 did not arise in the process due to both internal 

 and external factors. We know that abnor- 

 malities in growth do occasionally rise from ab- 

 normal shuffling of the units concerned. We 

 are not yet acquainted with the factors that 

 determine the limits of growth, but it is pro- 

 able that under certain conditions the limit of 

 growth is reached sooner in some cases than in 

 others. This in itself would produce variation. 



(2) Bisexual Reproduction. — In the higher plants and 



animals a new organism is the result of the 

 mingling of two inheritances in the male and 

 female germ cells (amphimixis), and as a result 

 must lead to the production of variation. The 

 offspring, therefore, tends to likeness with the 

 parents. 



(3) There is much scope for the variation in the intri- 



cate processes represented in maturation and 

 reduction. The loss of chromatic material 

 allows for different combinations of the heredi- 

 tary qualities. (See chapter 16) 



(4) Recent studies of the Slipper-animalcule {Para- 



mecium aurelia) by Woodruff and Erdmann 

 reveal periodic nuclear changes of a breaking- 

 down and a re-organizing nature. This pro- 

 cess is called endomixis, and is believed to be 

 regulatory in character as it secures the life of 

 the race indefinitely in the absence of conjuga- 

 tion. If such a process occur in the many- 

 celled animals may not the changes induce vari- 

 ations even in pure lines? 



Again in the regeneration of a Planarian 

 from a fragment, Child finds that a rejuvenes- 

 cence takes place, as measured by the increased 

 metabolism and power of resistance. May not 

 rejuvenescence take place in the germ cells 



29 



