3o6 EVOLUTION, GENETICS, AND EUGENICS 



In fact this whole compHcated system of indirect division is regarded 

 by most biologists as a mechanism for bringing about the precise 

 halving of the chromosomes. 



The chromosomes of each group at the poles finally fuse and two 

 new nuclei, each similar to the original one, are constructed (Fig. 44, 

 g and h). In the meantime a division of the cell-body is in progress 

 which, when completed, results in the formation of two complete 

 new cells. 



As all living matter, if given suitable food, can convert it into li\ing 

 matter of its own kind, there is no difficulty in conceiving how the 

 new cell or the chromatin material finally attains to the same bulk 

 that was characteristic of the parent cell. In the case of the chro- 

 matin, indeed, it seems that there is at times a precocious doubling 

 of the ordinary amount of material before the actual division 

 occurs. 



Chromosomes constant in number and appearance. — With some 

 minor exceptions, to be noted later, which increase rather than detract 

 from the significance of the facts, the chromosomes are always the 

 same in number and appearance in all individuals of a given species 

 of plants or animals. That is, every species has a fixed number which 

 regularly recurs in all of its cell-divisions. Thus the ordinary cells 

 of the rat, when preparing to divide, each display sixteen chromo- 

 somes, the frog or the mouse, twenty-four, the lily twenty-four and 

 the maw-worm of the horse only four. The chromosomes of different 

 kinds of animals or plants may differ very much in appearance. In 

 some they are spherical, in others rod-like, filamentous or perhaps of 

 other forms. In some organisms the chromosomes of the same nucleus 

 may differ from one another in size, shape, and proportions, but if such 

 differences appear at one di\'ision they appear at others, thus showing 

 that in such cases the differences are constant from one generation to 

 the next. 



Significance of the chromosomes. — -The question naturally arises 

 as to what is the significd,nce of the chromosomes. Why is the accur- 

 ate adjustment which we have noted for their division necessary ? 

 The very existence of an elaborate mechanism so admirably adapted 

 to their precise halving, predisposes one. toward the belief that the 

 chromosomes have an important function which necessitates the 

 retention of their individuaUty and their equal division. Many biolo- 

 gists accept this along with other evidences as indicating that in 

 chromatin we have a substance which is not the same throughout, that 



