HEREDITY, VARIATION AND DEATH 33 



bility is the inevitable result of the process of reproduction and 

 that one great function of Natural Selection is to strengthen 

 heredity and foster faithfulness to type. 



To find the cause of variations we must study the life of the A theory 

 simplest of known organisms : higher in the scale there is so cause O f 

 much complication that it is far more difficult to attain to clear variations 

 notions. In all the Protozoa we find multiplication by fission : death 

 an ultimate fact as far as we can see at present, the cause 

 of which we shall never be able to arrive at. But if fission 

 takes place, it is inevitable that it must at times be imperfect, 

 I*., that the two cells formed by the division of the nucleus 

 should remain adhering to one another. Thus we have, instead 

 of a one-celled Protozoon, a two-celled Metazoon. Here is an 

 enormous step onward and there is no reason why it should not 

 be often repeated, so that an animal similar to the Morula would 

 be formed. But for further progress variation in the cells them- 

 selves is necessary : there must be division of labour. When a 

 cell divides whether simply, or by the elaborate process called 

 Karyokinesis, which has been already described, 1 the two parts 

 appear to be exactly equal. But it is impossible that they 

 can always be exactly equal in size or exactly alike in char- 

 acter. As to size, mathematicians tell us that exact equality 

 is practically unattainable : as to character, the great compli- 

 cation of the organisation makes it impossible for the two 

 resultants to resemble each other in every detail. Hence 

 fission in itself is a source of variation. This Weismann saw 

 and, in one region, turned to account. To explain the advance 

 of evolution beyond the morula stage, in which the cells 

 are all alike, he assumed that after a few cell-divisions in 

 which all the characters were shared by all the resultants, 

 one or more cells must begin dividing unequally the characters 

 or determinants not being the same in the two halves. In this 

 the most thorough-going specialisation and all the complexity 

 of animal organisation have their origin. The soma with all 

 its varied powers is the achievement of the principle of unequal 

 specialising fission : equal fission, the resultant halves being 



i See p. 14. 



c 



