346 THE EVOLUTION THEORY 



of the body, which it does as a matter of course, then it follows that 

 after the reduction each germ-cell must either contain only half the 

 primary constituents of the parents or all the primary constituents 

 must be contained in the half number of chromosomes. The latter 

 seems to me the only possible assumption, as I shall immediately 

 proceed to show, and this is as much as to say that the primary 

 constituents of at least two complete individuals must be contained 

 in the chromosomes of the segmentation nucleus. 



That this conclusion is correct is obvious from the fact that 

 a whole, that is, a perfect individual with all its parts, develops 

 from the ovum, and not a defective one. For suppose that each 

 mature germ-cell contained only half the primary constituents of 

 the body, it would be impossible that these halves should always 

 exactly complete themselves to form a whole embryo when they 

 are brought together in fertilization, after having been halved 

 by mere chance during the preceding reducing division ; it would 

 be much more likely to happen that they did not complete themselves, 

 and that their union would therefore result in an individual with 

 certain parts wanting. If, for instance, in the sperm-cell only the 

 anterior half of the body was potentially present, and this united 

 with an ovum which likewise contained only the primary constituents 

 of the anterior half, the embryo resulting from their union would 

 lack the posterior half of the body, and so on. Of course so rough 

 a division of the primary constituents is not to be thought of, but 

 however fine we can imagine the halving of the mass of primary 

 constituents to be, there would never be any guarantee that the 

 two cells uniting in amphimixis would complete the mass of primary 

 constituents again ; indeed, the chance that the two exactly com- 

 plementary halves of the mass would meet would rather become 

 less the finer and more complex one imagines the halving by reducing 

 divisions to be. A perfect embryo with all its parts would rarely 

 arise, but now one group of parts, now another would be wanting, 

 while another group might be developed double, or at least would be 

 doubly present in the primary constituents. 



But in addition to this the facts of inheritance show us that 

 the resemblance to mother and father may express itself simul- 

 taneously in all the parts, or at least in the same parts of the child, 

 as may be seen with especial clearness among plant-hybrids, and 

 thus the conclusion is inevitable that even in the half number 

 of chromosomes all the primary constituents of the whole body are 

 present. 



Let us go a generation further. If the species possess four 



