VARIATION CONSIDERED BIOLOGICALLY. 187 



practically nil — for lie expressly states that he is not in a jiosition to " give 

 a proof in support of this supposition." Now Mr. MerrilieM lias jiroved 

 to a certain extent that these forms are interchangeable u})on the applica- 

 tion of the right temperature, hat is, that the spring form levana isobtained 

 with a \ow, irr or s a with a high temperature, applied at the time of pupal 

 development. It must be evident at the same time, that whatever varia- 

 tions there may be in any one germ, tending to produce either of these 

 forms, must be in every germ, and that, given a fit environment, levana 

 may become pvorsa, and prorsa may become levana. This being so, it 

 follows that such germ probal)ilitics as may exist, ai'e transmitted until 

 the pupal stage, for it is not until the pupal tissues have undergone 

 histolysis and are undergoing rehabilitation that the external factor — 

 temperature — comes into play. The possibility of eitlier form being 

 evolved, then, exists in the pujja, and we find that given an environ- 

 ment including a low tempei'ature the red-coloured levana is produced, 

 given one including a high temperature the dark-coloured prorsa. 

 Natural selection has given this species a distinct tendency to produce 

 special forms at special seasons under differences of pupal environment, 

 but ex})eriment has proved that the possibilities of becoming either are 

 in each pupa, with a strong tendency, however, in favour of producing 

 its own form. I would now ask — What has the ajipearance of the 

 dark Vaneasa prorsa, at the same time as a somewhat similarly coloured 

 Lhnenitis, to do with the biological principles involved in this change 

 of colour, which, as we have seen, appears to be largely governed by 

 an external stimulus —temperature ? Is it that Vanessa and Limenitis 

 are so closely allied (and they are very closely allied) that they both 

 still show traces in their summer forms of their common origin ? This 

 I can understand. Is it that this real relationship is accentuated by 

 the fact of Limenitis sibylla and FaHfssaj:irorsa being naturally subjected 

 to similar conditions of environment (temperature, etc.), in their pupal 

 stages, which different experimenters have proved to be the particular 

 stage in which V. prorsa is affected ? This would appear to be so, for 

 Weismann says, '' they fly together in open spaces in woods." If so, are 

 the mere facts connected with their similarity sufficient data to justify us 

 in assuming that this similarity is the cause of the summer appearance of 

 V.jyrorsa? Is this not, just a little, putting the cart before the horse? It 

 appears to me, that there is a very simple biological explanation of the 

 phenomenon, and that it is not after all so far removed from the pheno- 

 menon presented in the coloration of Chri/sophanus phloeas, which we 

 have just considered. 



It is very evident — without going into the argument as to whether 

 levana or prorsa is the older form — from the experiments on this 

 species, that every egg laid by the species, may, under certain conditions 

 of environment and external stimuli, develop either prorsa or levana. 

 The scale colour-determinants or possibilities then, from the very first, 

 range between the red of levana and the almost black oi prorsa, and it 

 seems to me that, since these same i)ossibilities exist until the very 

 time that the scales and their contained ^^ignient are finally formed in 

 the pupa just before emergence, the explanation lies in the intensity 

 or completeness with which the colour is developed and produced at 

 this stage. In other words, it is due to the ability with which the 

 environment allows the vital forces to make the most of the material 



