4-16 ORIGINATIVE FACTORS IN EVOLUTION: 



terial basis of inheritance, we may regard it as certain that 

 the chromosomes play an exceedingly important role as ve- 

 hicles of the heritable qualities. We may compare them to a 

 microscopic pack of cards and we know that they are some- 

 times visibly different from one another in the same germ- 

 cell, and that there is an extraordinarily elaborate shuffling 

 of the cards before development begins. In the reduction- 

 process involved in the maturation of almost every animal 

 egg-cell, half of the ovum's pack is thrown away, usually in 

 the first polar body, and comes to nothing. In the matura- 

 tion of the sperm-cell there is also a halving of the pack, 

 but all the reduced units are in this case functional. In 

 fertilisation the two half-packs come together in intimate 

 and orderly union, though without fusion of chromosomes, 

 forming the zygote-nucleus. The opportunities for permuta- 

 tions and combinations of hereditary items, and for the 

 dropping out of one or more altogether, are many and actual. 

 Thus the origin of variations of a quantitative sort does not 

 seem beyond our comprehension, except in the sense that we 

 do not in any way understand the process of cell-division, 

 whether meiotic or reducing division in the maturation of 

 the germ-cells, or the ordinary equational division in other 

 cases. 



That this is still only nibbling at the problem is evident 

 when we think of meristic variations (in the number of 

 parts, segments, vertebrae, joints, etc.), which Professor Bate- 

 son has usefully distinguished from substantive variations (in 

 the composition of materials). A re-shuffling of the molec- 

 ular cards within the germ-cell might give rise to a new 

 pigment which was continued in subsequent generations as a 

 definite constituent particle (which we have to credit with 

 great capacity for increase) or as a particular chemical 



