THE PKKSKNT POSITION OF THE THEORY OF OKOANIC EVOLUTION. 10.'] 



In attempting to give a history of life on the earth on these 

 lines we are thus led into a complete cul-de-sac. The continual 

 shuffling of potentialities brought about by the sexual union of 

 two germs we can understand, but how all these separate races 

 arose is left in insoluble mystery. 



It is interesting to recount the solution of the difficulty 

 which was offered by Weismann. Weismann had arrived at 

 s< mewhat similar conclusions to the Mendelians on totally 

 different grounds. His reasoning was as follows : In the case 

 of animals the two germs brought together in sexual union are 

 of different kinds ; one of these, the male germ, is very minute, 

 and the other, the female germ or egg, is much larger. Yet the 

 hereditary qualities of the progeny resulting from the union of 

 two varieties is just the same whichever variety supplies the 

 egg. Therefore the two germs so different in appearance must 

 be entirely alike in their hereditary potentialities. Now the 

 portion of the male germ which penetrates the female germ 

 consists entirely of the nucleus, and the conclusion is obvious 

 that the nucleus must be the bearer of the hereditary qualities. 

 But the nucleus is a complex structure ; it consists of a firm 

 wall enclosing a clear sap traversed by transparent cords called 

 linin, on which are strung a certain number of granules termed 

 chromatin, from their power of absorbing and holding staining 

 materials. When the nucleus divides, this chromatin arranges 

 itself in the form of a number of short, thick rods, called 

 chromosomes, and each chromosome becomes split longitudinally 

 and the two halves go to the two daughter nuclei. The number 

 of chromosomes produced in each dividing nucleus is the same, 

 and is characteristic of the species of animal to which the 

 nucleus belongs. Since then we find here a substance which 

 Nature takes the greatest pains to divide into precisely equal 

 halves at each division, and since the hereditary substance is 

 somewhere in the nucleus, Weismann jumps to the conclusion 

 that the chromatin is the hereditary substance of which he is 

 in search. Before the germ-cells are ready to unite each germ- 

 cell has only half the normal number of chromosomes. 

 Weismann assumes that the halving can take place in a random 

 manner, and thus he comes to the conclusion that at each 

 sexual union there is a reshuffling of chromosomes, and in this 

 way he accounts for the origin of inheritable variations. Then, 

 of course, he stumbles against the difficulty of accounting for 

 the different inheritable qualities embodied in the different 

 chromosomes. Weismann assumes that these differences began 

 in the simple ancestors of the higher animals and plants, which, 



