And now we come to the additions which I indicated at the outset as having to be made—certain evidences which have come to light since this controversy commenced.
When, by a remembered observation made in boyhood, joined with the familiar fact that worker-larvæ can be changed into the larvæ of queens by feeding, I was led to suggest that probably all the variations of form in the social insects are consequent on differences of nurture, I was unaware that observations and experiments were being made which have justified this suggestion. Professor Grassi has recently published accounts of the food-habits of two European species of Termites, shewing that the various forms are due to feeding. He is known to be a most careful observer, and some of the most curious of his facts are confirmed by the collection of white ants exhibited by Dr. David Sharp, F.R.S., at the soirée of the Royal Society in May last. He has favoured me with the following account of Grassi's results, which I publish with his assent:—
"There is great variety as to the constituents of the community and economy of the species in White Ants. One of the simplest conditions known is that studied by Grassi in the case of the European species Calotermes flavicollis. In this species there is no worker caste; the adult forms are only of two kinds, viz., soldiers, and the males and females; the sexes are externally almost indistinguishable, and there are males and females of soldiers as well as of the winged forms, though the sexual organs do not undergo their full development in any soldier whether male or female.
"The soldier is not however a mere instance of simple arrested development. It is true that there is in it arrested development of the sexual organs, but this is accompanied by change of form of other parts—changes so extreme that one would hardly suppose the soldier to have any connection with either the young or the adult of the winged forms.
"Now according to Grassi the whole of the individuals when born are undifferentiated forms (except as to sex), and each one is capable of going on the natural course of development and thus becoming a winged insect, or can be deviated from this course and made into a soldier; this is accomplished by the White Ants by special courses of feeding.
"The evidence given by Grassi is not conclusive as to the young being all born alike; and it may be that there are some individuals born that could not be deviated from the natural course and made into soldiers. But there is one case which seems to show positively that the deviation Grassi believes to occur is real, and not due to the selection by the ants of an individual that though appearing to our eyes undifferentiated is not really so. This is that an individual can be made into a soldier after it has visibly undergone one half or more of the development into a winged form. The Termites can in fact operate on an individual that has already acquired the rudiments of wings and whose head is totally destitute of any appearance of the shape of the armature peculiar to the soldier, and can turn it into a soldier; the rudiments of the wings being in such a case nearly entirely re-absorbed."
Grassi has been for many years engaged in investigating these phenomena, and there is no reason for rejecting his statement. We can scarcely avoid accepting it, and if so, Professor Weismann's hypothesis is conclusively disposed of. Were there different sets of "determinants" for the soldier-form and for the winged sexual form, those "determinants" which had gone a long way towards producing the winged sexual form, would inevitably go on to complete that form, and could not have their proclivity changed by feeding.
[Yet more evidence to the like effect has since become known. At the meeting of the Entomological Society, on March 14, 1894 (reported in Nature, March 29):—
"Dr. D. Sharp, F.R.S., exhibited a collection of white ants (Termites), formed by Mr. G. D. Haviland in Singapore, which comprised about twelve species, of most of which the various forms were obtained. He said that Prof. Grassi had recently made observations on the European species, and had brought to light some important particulars; and also that in the discussion that had recently been carried on between Mr. Herbert Spencer and Prof. Weismann, the former had stated that in his opinion the different forms of social insects were produced by nutrition. Prof. Grassi's observations showed this view to be correct, and the specimens now exhibited confirmed one of the most important points in his observations. Dr. Sharp also stated that Mr. Haviland found in one nest eleven neoteinic queens—that is to say, individuals having the appearance of the queen in some respects, while in others they are still immature."
Another similarly conclusive verification I published in Nature for December 6, 1894, under the title "The Origin of Classes among the 'Parasol' Ants." The letter ran as follows:—