XVI ARE ACQUIRED CHARACTERS INHERITED ? 339 



For here we have a species, the larvae of which for 

 thousands, perhaps millions of generations, have fed upon 

 one species of plant, and the perfect insect has a definite 

 set of characters. But when the larvae are fed on a 

 distinct but allied species of plant, the resulting perfect 

 insect differs both in colouration and form. We may 

 conclude from this fact, that some portion of the characters 

 of the Texan species were dependent on the native food- 

 plan b, Juglans nigra, and that this portion changed under 

 the influence of the new food-plant. Yet the influence of 

 the native food had been acting uninterruptedly for 

 unknown ages. Why then had the resulting characters 

 not become fixed and hereditary ? The obvious conclusion 

 is, that, being a change produced in the body only by the 

 environment, it is not hereditary, no matter for how many 

 generations the agent continues at work ; in Weismann's 

 phraseology, it is a somatic variation, not a germ- 

 variation. 



The other case is that of the brackish water shrimp, 

 Artemia salina, which, by the water becoming gradually 

 Salter, was changed into what had before been considered 

 a distinct species, A. milhausenii ; and the reverse change 

 was also effected, the modification in both cases being in 

 proportion to the alteration in the salinity of the water, 

 and therefore spread over two or three generations. Here 

 too there seems to be no heredity, however long either 

 form has been submitted to the influence of the modifying 

 medium. 



Similar to this is the case given by Mr. Thiselton 

 Dyer {Nature, vol. xliii., p. 581) of the tissue-papery 

 leaved Arahis anachoretica, which grows in hollows of 

 rocks sheltered from sun and rain, but which, when 

 cultivated at Kew, changed at once to the common Arahis 

 alpina, of which it is a modification due to the action of 

 the environment on the individual plant. How different 

 is this from the behaviour of plants which have been 

 developed by germ-variation and selection, such as most 

 of the true alpine plants which retain their compact 

 dwarf foliage and large flowers under very different 

 conditions when raised from seed in our gardens ! Other 



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