Page:Proceedings of the Royal Society of London Vol 69.djvu/211

Rh dealt with if it can l>e detected ; but if differentiation occurs in different individuals in different degrees and directions, how can it be told whether the ensuing deviations in correlation are due to a change in the control of individuality over the variation, or to irregular and incipient differentiation ? Yet, is not such differentiation exactly what is to be expected in the variation of homotypes ? Do not most animals and plants exhibit this phenomenon, and must we not believe that these organisms have attained their present forms largely by variations among their repeated parts ? In view of these familiar facts, can Professor Pearson point to any feature which positively dis- tinguishes variation occurring between members of a series from differentiation ?

That differentiation may in practice be mistaken for variation between homotypes he is aware. It is not, however, the difficulty of recognition I would now emphasise, but the fact that between the two phenomena no absolute distinction exists in nature. An " undiffer- entiated series of like parts " means only a series of like parts which have varied and are varying among themselves but little. A series of highly variable like parts is a series in which differentiation exists or is beginning to exist in a complex and irregular fashion. A " differentiated series of like parts " means a series among which variation is or has become definite and regular. Between these classes there is every shade and degree. No one can say finally where each begins and ends, and, by appropriate selection, we could find homotypic coefficients of any required value. The average value of such coefficients taken at random has no significance in nature.

Let us examine some practical examples.

In Professor Pearson's Nigella, for example, the correlation between the numbers of segments in the capsules of individual plants is found to be low. That is to say, given one seed-vessel of the plant, it will give you very little information as to the most probable number of segments in a second seed-vessel of the same plant. Why is this ?

From the look of the plant, or, if such simple perceptions are mistrusted, by counting the segments of seed-vessels on lateral branches, and com- paring the numbers obtained with those obtained from seed-vessels borne on central axes only, it is easy to show, as Professor Pearson points out, that the numbers are generally lower in the case of the laterals. We recognise, further, that the proportion of laterals varies from plant to plant.

How is the differentiation detected in Nigella? By the regularity with which small capsules are associated with lateral branches.

But suppose that for any reason this regularity were masked, should we then perceive the differentiation ? Might it not pass, wholly unsuspected, for a change in correlation ? Undoubtedly it might.

Take the case of blood-corpuscles of a Frog. Measure some charac-