No. 584] HEREDITY AND ITS MEANING 481 



some pairs, is considered to be very constant in its char- 

 acters from the taxonomist's standpoint, yet by careful 

 continued observation Morgan has succeeded in detecting 

 over 130 mutations. 



From a strictly mathematical standpoint, it would seem 

 that if other things are equal, variability would take place 

 in proportion to the number of chromosome units. The 

 difficulty is that in no case do we know anything whatever 

 about the relative complexity of any particular chromo- 

 some unit. One must infer, however, that the 47^8 chro- 

 mosomes in man are individually much more complex than 

 the 128-132 chromosomes in the fern Nephrodium molle. 

 If this inference be correct there are reasons why altera- 

 tion in determinants may occur in direct proportion to 

 the number of chromosomes or rather to the mass of chro- 

 matin without there being visible somatic variability in 

 the same ratio. Let us construct an imaginary plan for 

 preventing visible variation without preventing change 

 in chromosome determinants. Unquestionably the sim- 

 plest means is to double the chromosome number. Se- 

 lecting, for example, a species with four chromosomes, let 

 us suppose that fertilization occurs without a reduction 

 division at some time or other. Then instead of a dual 

 organism with two sets of chromosomes of similar func- 

 tion, one from the male and one from the female parent, 

 there would be a quadruple organism with two sets of 

 similar chromosomes from each parent. Any germinal 

 change which would produce a iieiv dominant character 

 would be apparent immediately, but for a recessive change 

 to appear— and these are many times as numerous as the 

 others— it would be necessary to have identical changes 

 occur in two chromosomes. Following out this line of 

 reasoning, it is not hard to see what a great possibility 

 for uniformity there is in further chromosome duplication, 

 provided the actual fact of duplication makes no great 

 change in the organism. That chromosome doubling has 

 no decided visible effect we have seen from the cases 

 already described ; and since so many nearly related spe- 



