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the same^ it was easy to believe that in the course of time all 
these difFerent things became part of the inherited set. But this 
is obviously an unnecessary assumption. If^ to take an example, 
in an organism there exist organs, wliicli are so constituted that, 
under the influence of a grouping of certain transmitted non- 
miscable substances, under the influence of gravitation, it reacts 
by taking a certain position in respect to the vertical, I for one 
do not see the necessity, even granted the possibility, of this 
effect of gravitation becoming hereditary. The earth is always 
under all these organisms to attract them. 
If we find that two species of the genus Mus, one living 
in our parts, the other in the tropics, differ amongst other things 
in taillength, it might be said by a Neo-Lamarckist that 
here the effect of temperature at least had become hereditary. 
But after the experiments of Przibram we know that it is 
unnecessary to make this assumption, the constantly higher tem- 
perature in the tropics causing all the individuals of the species 
to have long tails. One could only compare the genetic factors 
in the taillength of these animals, by growing them for a gene- 
ration in identical temperature. 
The colours of butterflies are due to a number of factors, 
among others a certain temperature during the pupal stage. Only 
if pai^ent and ofi'spring be subjected to the same temperature are 
they identically coloured, Loeb has suggested that the diff'erently 
coloured patches might have a difi"erent temperature-coefi'icient of 
development. In such a case, the effect of temperature does not 
become fixed, it is not fixable, and it would not be of any use 
for the organism if it were. Undoubtedly, such examples are very 
numerous. For instance in some birds, pigmentformation requires 
a certain minimum temperature, below which their feathers are 
produced pigmcntless. Probably it can be said that the tempera- 
ture-coefPicient for the growth of their feathers is smaller than 
that for the formation of pigment. Such birds, like the ptarmigan, 
the razor-bill, the guillemot will be lighter-coloured in winter 
than in summer. In the reverse case, when the tcmi)erature 
coefficient of feathergrowth is greater thau that for i)ignient- 
formation, the birds must be lighter in summer. Such is ol)- 
viously the case with the snow-bunting, whicli is whiter in summer 
than in winter. In mammals we have some examples of the tirst 
sort like the stoat and the weascl, and some o\' tlie second sort. 
