Page:Proceedings of the Royal Society of London Vol 69.djvu/348

332 complex of characters, certain functions of those quantities remain constant, and such constants ought to be discoverable, at any rate in theory, and should serve as the criterion of a common origin, when we deal with local races as having been subjected only to a selection directly differentiating a comparatively few characters.

In this memoir the analysis is confined to the case of normal fre- quency, but most of the chief results are true for all cases of regression. The effects of selection are illustrated in a very consider- able variety of cases, especially the influence of selection on the coeffi- cients of heredity is fairly fully dealt with. Tables are given for the simpler cases to enable the biologist at once to appreciate the influence of selection, not only on the size and variability of organs, but on their correlations.

If selection has changed a race from a condition A to a condition B, it becomes of much interest to determine the nature of the selective death-rate by which the process has been carried on, and it is found that this death-rate as represented in the surface of survival-rates enables us to distinguish two kinds of selection, termed in the memoir positive and negative selection. In the first case a race is modified, because the nearer its members are to having their organs with a certain system of values, the better fitted they are to survive ; in the second case the nearer the individuals are to this system the less fitted they are to survive. There will usually be in this second case, not a single system, but an indefinite number of systems which would equally well fit individuals to survive ; in the first case, on the other hand, there are an indefinite number of systems which equally unfit their owners for surviving. This distinction seems of considerable interest.

For example, to select from the French race a race in femur and humerus like the Aino, we should have to proceed by a positive selec- tion ; but to select from the Aino a race like the French, we should have to proceed by a negative selection. To get 1000 Aino we should have to select for these two organs alone out of some 6,000,000 Frenchmen, but to get 1000 Frenchmen from the Aino we must select from about a billion of the latter. Thus we are to some extent able to appreciate the stringency of the selection, which even lasting through long ages, and introducing continuous reproduction, would be needful to enable us to pass in the case of only two organs from one race to the other. Another point brought out by the surface of survival-rates is the fact that the fittest to survive are usually not the most frequent survivors.

It will be seen that the memoir opens up a novel field of investiga- tion, but one so wide that the theory of it must be limited by close contact with what is needful for the purposes of evolution. We want measurements on the local races of animals to guide us ; at present we