368 
De Vries on the Origin of Species 
(96 huurs old) has practically no amnion at the sides, and none at all behind; but 
the head projects into an amniotic sack, which is widely open posteriorly. Now 
under a wide range of conditions, including all the differences between develop- 
ment within a uterus or within an egg-shell, the chance that a bird, or reptile, or 
mammal will not develope an amnion is of about the same order as the chance 
that it will not develope a head ; and the amnion is probably the less variable 
structure of the two. The production of an amnion is a phenomenon which I 
think Professor de Vries will certainly not include in his category of individual 
variations, and yet it can be completely suppressed during the life of a single 
individual by changing the appropriate group of external conditions. Until we 
know far more than we know at present about the relation between an organism 
and its environment, it is simply useless to discuss the stability of characters, 
whether " variations " or " mutations," except under environmental conditions 
which are as constant as we can make them during the period under discussion. 
The characters which give their value to the improved races of wheat, and to 
many of our cultivated plants, are admittedly in lai-ge part the direct i-esult of 
cultivation under special conditions ; and in order to judge whether the effect of 
selection on such plants is permanent we must grow them without selection under 
the same carefully arranged conditions of nutrition as those adopted for the culture 
of the race during the operation of selection. The evidence as it stands gives little 
or no indication what their behaviour under such circumstances would be. 
Apart from cases in which tlie cessation of selection has been accompanied by 
a change in the conditions of culture, the proof that selected varieties are unstable 
is actually made to include cases in which selection has been deliberately reversed, 
such as those of Buckman and Watson on parsnips and cabbages, quoted by 
Darwin. 
For the reasons indicated, the discussion of the facts relating to the stability of 
selected races, given by Professor de Vries, seems to me to be largely irrelevant. 
The view that the focus of regression in the offspring of a merely variable 
species is constant is substantially that adopted as a limiting hypothesis by 
Professor Pearson in 1895*. Professor Pearson at that time put forward two 
limiting hypotheses ; one that the focus of regression is as Professor de Vries 
supposes, fixed, the other that the focus of regression changes with every genera- 
tion. The whole of the work since published, both by Mr Galton and by Professor 
Pearson and his pupils, goes to show that the focus of regression, for each generation, 
is its oiun mean : hence the array of offspring, produced by parents who differ by a 
fixed amount from the mean of the parental generation, will have a mean deviation 
from the mean of the whole filial generation such that the ratio 
mean deviation of offspring from filial mean 
deviation of parents from parental mean 
will be equal to the coefficient of regression. 
* "Mathematical Contributions to the Theory of Evolution, III." Phil. Trans., A, 1895, 
pp. 253—318. 
