32 GENETIC VARIATIONS 



taining two chromosomes of that pair, we get an indi- 

 vidual that has four chromosomes belonging to one of its 

 pairs ; the genes of these chromosomes are thus in fours. Such 

 individuals have different characteristics from any of the 

 others of which we have spoken. There are in the Jimson 

 weed twelve different varieties obtainable by giving four 

 chromosomes in place of two, in different cases, to each of 

 the twelve chromosome pairs. And sixty-six more types can 

 be formed by giving four chromosomes to each of two of the 

 pairs, in each of the sixty-six different combinations of two 

 from twelve. 



Thus the number of diverse types of organisms producible 

 in these ways is becoming large. We have thus far enumerated 

 two hundred and thirty-four possible types. It might appear 

 that we could go on multiplying varieties indefinitely in this 

 way, by adding or subtracting one or two, to each of three 

 pairs, then to each of four pairs and so on. But nature fails us 

 when this is attempted. If more than two pairs are thus 

 changed by addition or subtraction, yet not all the twelve are 

 thus changed, the individual cannot live; it seems then to be 

 unbalanced and so incapable of development. 



Yet if all the chromosome pairs lack one chromosome — or 

 if all have one or two additional chromosomes — then all be- 

 ing changed equally, there is no lack of balance, so the indi- 

 viduals do not die, but live and develop. Our diagram of 

 Figure 7 shows various different types that are producible in 

 these ways. In the diagram there are represented but four 

 pairs of chromosomes instead of the twelve of Datura. The 

 normal individuals, known as diploids (B), have four pairs. 

 Others are known in many organisms in which the chromo- 

 somes are single instead of paired; these are known as hap- 

 loids. Others have three chromosomes of each kind (in place 

 of but two or one); others have four of each kind (tetra- 



