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to show that changed conditions act in two ways, directly on the whole organization or on certain parts alone, and indirectly through the reproductive system. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. In all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions. The direct action of changed conditions leads to definite or indefinite results. In the latter case the organisation seems to become plastic, and we have much fluctuating variability. In the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all, or nearly all the individuals become modified in the same way. ↑
It is very difficult to decide how far changed conditions, such as of climate, food, &c., have acted in a definite manner. There is some reason to believe that in the course of time the effects have been greater than can be proved to be the case by any clear evidence. ↑ But we may safely conclude that the innumerable complex co-adaptations of structure ... which we see .. throughout nature between various organic beings, cannot be attributed simply to such action. In the following cases the conditions seem to have produced some slight definite effect: E. Forbes asserts that shells at their southern limit, and when living in shallow water, are more brightly coloured than those of the same species from further north or from a greater depth; but these statements have lately been disputed. Mr. Gould believes that birds of the same species are more brightly coloured under a clear atmosphere, than when living on islands or near the coast; and Wollaston is convinced that residence near the sea affects the colours of insects. Moquin-Tandon gives a list of plants which when growing near the sea-shore have their leaves in some degree fleshy, though not elsewhere fleshy. .. Other similar facts could be given.
The fact of varieties of one species, when they range