INTERNAL CAUSES OF VARIATION 175 



it may be well to note the substance of what is really known 

 about their actual constitution. 



That they are not masses of homogeneous matter is certain, 

 and that they consist of numbers of small granules capable of 

 multiplication and division seems equally certain. To quote 

 Wilson : 



The facts are now well established (i) that in a large number of cases 

 the chromatin thread consists of a series of granules (chromomeres) 

 imbedded in and held together by the linin substance ; (2) that the split- 

 ting of the chromosomes is caused by the division of these more elementary 

 bodies ; (3) that the chromatin grains may divide at the time when the 

 spireme is only just beginning to emerge from the reticulum or resting 

 stage. 



Because of these facts there arises the strongest tendency to 

 attach individuality to the chromatin granules and to conceive 

 them as built up of definite, though often diverse, physiological 

 units, thus constituting a semi-mechanical basis for heredity, and 

 incidentally for variation as well. This assumption Weismann 

 and others have made. Whether the facts should be pushed to 

 this extreme interpretation is, in the opinion of the author, as 

 yet uncertain. The facts are extremely suggestive, to say the 

 least, and it is certainly not too much to believe that at this 

 point we have touched the physical basis of life and in some 

 fashion the very root of inheritance and variation ; indeed we 

 may proceed upon the conviction that transmission is a function 

 of the chromatin granules. 



Reduction as a cause of variation. The most remarkable and 

 suggestive fact about living beings is the numerical constancy 

 of chromatin units (chromosomes) for each species, and the 

 most remarkable and suggestive of all the vital processes is 

 their reduction before fertilization. If, as we suppose, the 

 chromosomes are the physical basis of inheritance, then in the 

 loss of chromatin matter at maturation lies a fimdamental cause 

 of variation, and one quite independent of the effects of fertili- 

 zation afterward. 



Reduction would seem to be a process calculated to insure 

 that no two germ cells, even from the same -individual, should 

 ever be alike, and this is the most evident reason for the 



