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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, wliich 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 Przibrara 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 parent and offspring be subjected to the same temperature are 

 they identically coloured. Loeb has suggested that the differently 

 coloured patches might have a different temperature-coefficient 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 pigmentless. Probably it can be said that the tempera- 

 ture-coefficient 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 temperature 

 coefficient of feathergrowth is greater than that for pigment- 

 formation, the birds must be lighter in summer. Such is ob- 

 viously the case with the snow-bunting, which is whiter in summer 

 than in winter. In mammals we have some examples of the first 

 sort like the stoat and the weasel, and some of the second sort, 



