In the simple case where the homozygous and heterozygous conditions are indistinguishable in appearance the number of possible forms is 2, raised to the power of the number of factors concerned. Thus where one factor is concerned there are only 2^1 = 2 possible forms, where ten factors are concerned there are 2^{10} = 1024 possible forms differing from one another in at most ten and at least one character. Where the factors interact upon one another this number will, of course, be considerably increased. If the heterozygous form is different in appearance from the homozygous form, there are three possible forms connected with each factor; for ten such factors the possible number of individuals would be 3^{10} = 59,049; for twenty such factors the possible number of different individuals would be 3^{20} = 3,486,784,401. The presence or absence of a comparatively small number of factors in a species carries with it the possibility of an enormous range of individual variation. But every one of these individuals has a perfectly definite constitution which can {137} be determined in each case by the ordinary methods of Mendelian analysis. For in every instance the variation depends upon the presence or absence of definite factors carried in by the gametes from whose union the individual results. And as these factors separate out cleanly in the gametes which the individual forms, such variations as depend upon them are transmitted strictly according to the Mendelian scheme. Provided that the constitution of the gametes is unchanged, the heredity of such variation is independent of any change in the conditions of nutrition or environment which may operate upon the individual producing the gametes.