(d.) Elongation accompanying rises of internal pressure.
(e.) Pulsatile increase of volume during sudden brief elevations of internal pressure.
The discordant results obtained by various observers (Marey, Roy, and others) as regards the distensibility of arteries, are in large measure explicable by the absence or presence (in varying degree) of post- mortem contraction in the arteries they used in their experiments.
In his paper on " Honiotyposis,"* of which an abstract appeared in the ' Roy. Soc. Proc.,' vol. 68, p. 1, Professor Pearson raises an issue of extra- ordinary importance. In any attempt to perceive the true relation of variation to differentiation, and to analyse the essential similitude existing between Heredity and Repetition of Parts, we reach a funda- mental problem of biology. Little has thus far been done towards eluci- dating this problem or even towards formulating it. The appearance of Professor Pearson's remarkable memoir may perhaps therefore with profit be taken as an occasion for considering critically some aspects of these questions.
It is impossible to write of Professor Pearson's paper without expressing a sense of the extraordinary effort which has gone to its production and of the ingenuity it displays. But on careful examina- tion it will, I think, be seen that in the light of known facts there is. serious doubt whether the determination of what Professor Pearson calls the average homotyposis of " undifferentiated like parts " can be attained by his observations, and that there is even graver doubt whether, if it was attainable, such a value would have any natural significance. In the course of this consideration it must, I think, also appear that the comparison he attempts between the average homotyposis of " undifferentiated like parts " and average fraternal correlation in families is incorrectly instituted.
At the outset I wish to express the conviction that the leading idea which inspired and runs through the work is a true one. Professor Pearson suggests that the relationship and likeness between two brothers is an expression of the same phenomenon as the relationship and likeness between two leaves on the same tree, between the scales on a moth's wing, the petals of a flower, and between repeated parts
- ' Phil. Trans.,' A, 1901, vol. 197, p. 285.
