Page:Popular Science Monthly Volume 62.djvu/281

Percentage of each Type in first six Generations of the Dihybrid
VB X GS.

		First.	Second.	Third.	Fourth.	Fifth.	Sixth Genera tion.
VS	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	each 0	6.25	14.06	19.10	21.97	23.46
VB
GS
GB
VgS	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	each 0	12.50	9.38	5.47	2.93	1.15
VqB
VSb
GSb
VgSb		100	25.00	6.25	1.56	.39	.10

Hence, if we sow all the seed of each generation, it is seen that each of the homozygote types approaches 25 per cent, of the whole, while all the heterozygote types approach zero, and the larger the number of latent characters in a type, the more rapidly it decreases in proportion. In trihybrids, we should have eight homozygote types, each increasing toward 12¹⁄₂ per cent. of the whole. Hence the generalization, a hybrid of the nth order tends to split up into 2n fixed types, all types not fixed tending to disappear. The effect of such a law, in the case of accidental hybrids between species and varieties in the wild state, can not fail to be important in the evolution of species. I leave the discussion of this interesting phase of the subject till the law is more generally confirmed. In this connection it may be well to state that, at the recent international conference of plant breeders in New York, Professor Bateson asserted that Mendel's law has been found to hold in every case where it had been thoroughly tested.^[1] The groups in which these tests have been made are so varied, representing both plants and animals, that the presumption in favor of the generality of the law is strong enough to warrant breeders in searching for it everywhere.

It has been urged by certain breeders that, even if the law is general, it can not be put to practical use by breeders; for it nearly always happens that the varieties crossed differ in an indefinite number of respects, and we should therefore get so vast a number of resulting types that no two individuals could be classed together. This objection is not altogether valid. In the first place, if we take any established variety and examine the individuals closely, we find no two of them alike. Hence, even if the variety we are trying to produce must consist of an indefinite number of types differing only in minor details, we are no more than duplicating the actual conditions existing amongst present useful varieties. In the second place, a very common problem of the breeder is to unite two characters found

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1. This does not agree with Correns (l. c.). I think, however, that the cases to which Correns refers may be explained by means of Mendel's law.