I am glad that the same answer has been made to Weismann's All-sufficiency of Natural Selection by two biologists, Herbert Spencer and Emery, simultaneously with mine. Emery, a specialist upon the structure of ants, and Herbert Spencer, relying upon the investigations of several Englishmen, have sought to prove that the differences between the individuals in the colonies of ants, bees, and termites, have been slowly called into existence by the operation of external influences affecting the egg in its situation and food during development.
It has been shown fully by experiment and by observation that the fertilised eggs of the queen bee may become either workers or queens. This depends merely on the cell in the hive in which the egg is placed, and on what food the embryo is reared. In the specially large cells, known as queens' chambers, and with specially nutritious diet, they become queens. With poor food, and in smaller cells, they become workers. Even if worker larvæ be supplied in time with a richer diet, they may be turned into queens.
Similarly, the differences that exist among termites and ants, as Emery shows, may be described as polymorphism due to food. The Italian zoologist, Grassi, has shown that termites have it in their power to alter the relative numbers of workers and soldiers, and to produce as many of the latter as may be required, and they are able to accelerate the sexual maturity of other individuals by supplying nourishment suitable for stimulating the maturation of the genital organs.
Emery explains this polymorphism by attributing it to the general laws of growth in the insect organism under the influence of different external stimuli. He thinks that 'the production of workers depends upon a special capacity of the germplasm to respond to the abundance or scantiness of certain nutritive materials by a greater growth of certain parts of the body, and a lesser growth of other parts. Workers' food stimulates growth in the jaws and brain, retards growth in the wings and sexual cells. Queens' food has the opposite action.' There is a correlation between retardation of the sexual glands and acceleration of the development of the head, just as in vertebrates there is a correlation between the sexual glands and the secondary sexual characters. 'The characters by which the workers differ from the queens, therefore, are not innate, but are produced secondarily.'
Quite independently, but simultaneously, Herbert Spencer has suggested the same explanation as Emery. Moreover, he has used the conditions that exist among the state-forming insects as a strong argument against Weismann's doctrine of determinants. The observations of many careful persons, such as Charles Darwin, Emery, and others, show that in many species of ants the extreme types of individuals are connected by many intermediate forms. (Apud Emery, this is the case in many Myrmicidæ, in most Camponotidæ, and in Azteca.) These forms are transitional, not only in general size, but in the degree to which the genital organs have been arrested, and in the peculiarities of the jaws.
Spencer explains these transitional forms, and I agree with him, by supposing that the stoppage in food supply has taken place at different times after development has begun. ('It must happen that the stoppage of feeding will be indefinite.') Thus, the existence of transitional forms presents no difficulty on the theory of the agency of food. But how can the doctrine of determinants be applied to it? 'If he is consistent' (says Spencer, Contemporary Review, lxiv., p. 901), 'he must say that each of these intermediate forms of workers must have its special set of "determinants," causing its special set of modifications of organs; for he cannot assume that while perfect females and the extreme types of workers have their different sets of determinants, the intermediate types of workers have not. Hence we are introduced to the strange conclusion that, besides the markedly distinguished sets of determinants, there must be, to produce these intermediate forms, many other sets slightly distinguished from one another—a score or more kinds of germplasm, in addition to the four chief kinds. Next comes an introduction to the still stranger conclusion, that these numerous kinds of germplasm producing these numerous intermediate forms are not simply needless, but injurious—produce forms not well fitted for either of the functions discharged by the extreme forms, the implication being that natural selection has originated these disadvantageous forms. If, to escape from this necessity for suicide, Professor Weismann accepts the inference that the differences among these numerous intermediate forms are caused by arrested feeding of the larvæ at different stages, then he is bound to admit that the differences between the extreme forms, and between these and perfect females, are similarly caused. But if he does this, what becomes of his hypothesis that the several castes are constitutionally distinct, and result from the operation of natural selection?'
My course of thought leaves me with little to add to this criticism by Spencer. In this case, as in many others that I have pointed out, Weismann makes his usual mistake. He incorporates in the rudiment what really are stimuli coming from external conditions during the process of development; he makes a grave confusion between the rudiment and the conditions of its development.
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.'