In my view, in these cases of polymorphism in the colonies of insects Nature exhibits a series of most important experiments, and their plain meaning is that the same germinal material, when subjected to different external influences, may produce very different final products. When from the neutral germinal material of an insect egg there is produced a male or female creature, or a worker or soldier (as this or that influence acts), the process is no other, and presents no greater difficulties, than when an experimenter, taking the young bud of a plant, according to the conditions to which he subjects it, can turn it into a vegetative or into a reproductive shoot, a thorn or a root; no different to what occurs when the investigator, cutting into a _Cerianthus_, produces a second or third mouth, surrounded by tentacles, or in the case of _Cione_ surrounded by eye-spots. It has been shown, I think, in these pages that much of what Weismann would explain by determinants within the egg must have a cause outside the egg. The chief factors in the process of development we have found to be: (1) The multiplication of cells by division (growth as a moulding factor); (2) the relations of cells to their external environment (position in its widest sense as a factor); (3) the interrelations of the parts of a whole (cells, tissues, and organs) to one another and to the whole (correlative development). There remains to be considered the extent to which the germinal material in the egg determines the course of development of the organism. Here, before all things, it must be insisted that the individual nature of the cell determines the specific fashion in which the cell will react to the varying stimuli coming from varying conditions. The same agency produces very different results upon different organisms. These differences must be attributed to the differences in the nature (different intimate structure) of the active material. Sachs speaks strikingly on this point (_Physiology of Plants_, p. 602): 'If the same external cause induces exactly opposite effects in the organs, the explanation of this must simply be sought in the different structure of the organs. If one organ, when illuminated from one side, becomes curved so as to be concave on the side turned towards the centre of light, while another becomes convex on that side, the cause can only lie in the internal structure of the organ. But it is just on such differences of structure that the great variety of reactions which the most different plant organs exhibit towards the same external influences depends; and, fundamentally, all that we term biology--the mode of life of organisms--depends upon the fact that different organisms react differently towards the same external influences, and these reactions differ not only qualitatively, but also quantitatively, the finest gradations existing in both cases.'