and other Conceptions of Biology. 197 



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 un 

 suspected, for a change in correlation ? Undoubtedly it might. 



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



VOL. LXIX. P 



