Lecturing and reviewing were, so to speak, pastimes to him, and gave him little trouble. One lecture given at the Royal Institution on 'The Mental Differences between Men and Women' drew upon the head of the unlucky lecturer a great storm of indignation—why, the writer of this memoir has never been able to discover.
In May 1886, Mr. Romanes read a paper before the Linnean Society on 'Physiological Selection, an additional suggestion on the origin of species.' This paper was the outcome of many years' study of the philosophy of evolution, during which time he had gradually been coming to the conclusion that natural selection cannot be regarded as the sole guiding factor in the production of species, but that there must be some other cause at work in directing the course of evolution.
The theory of natural selection rests on two classes of observable facts: first, that all plants and animals are engaged in a perpetual struggle for existence, there being in every generation of every species a great many more individuals born than can possibly survive; and secondly, that the offsprings, although closely resembling the parent form, do present individual variations. It follows, therefore, that those individuals presenting variation in any way beneficial to them in the struggle for existence will survive as being the fittest to do so, Nature, so to speak, selecting certain individuals of each generation, enabling them not only to live themselves, but also to transmit their favourable qualities to their offspring. If a special line of variation is in some way preserved, there may result a variety so fixed and so distinct from the parent and collateral related forms as to constitute a separate species.
Further, since the environment (i.e. the sum total of the external conditions of life) is continually changing, it follows that natural selection may slowly alter a type in adaptation to the slowly changing environment, and if in any case the alterations effected are sufficient in amount to lead naturalists to name the result a distinct species, then natural selection has transmuted one specific type into another.
Mr. Romanes pointed out that the theory of natural selection only accounts for such organic changes as are of use to the species—by use signifying life-preserving—that it is, in fact, a theory of the origin and cumulative development of adaptations, whether these be distinctive of species, or of genera, families, classes, &c.
The question then arises, do species differ from species solely in points of a useful character, as they undoubtedly should do if natural selection has been the sole factor in their formation? Investigation shows that systematists recognise a species by a collection of characters, the value of a character depending not on its utility, but upon its stability; in fact, a large proportional number of specific characters, such as minute details of structure, form, and colour, are wholly without meaning from a utilitarian point of view. Investigation further shows that the most general of all the 'notes' of a true species is cross-infertility, that is, the infertility of the offspring of two individuals belonging to separate species: this, it was urged, could not be due to the action of natural selection. Lastly, apart from the primary distinction of cross-infertility, and the inutility of so many of the secondary specific distinctions, neither of which can be explained by the action of natural selection, Mr. Romanes was strongly of the opinion that even if a beneficial variation did arise, the swamping effects of free intercrossing would reabsorb it, and so render evolution of species in divergent lines, as distinguished from linear transmutation, impossible. This last difficulty can only be met by assuming that the same beneficial variation arises in a number of individuals simultaneously, for which assumption our present knowledge furnishes no warrant. If natural selection is brought forward as the sole factor in the guidance of organic evolution, then he considered that these difficulties remain insurmountable; if, however, it is regarded as a factor, even the chief factor, then these difficulties vanish, it being consistent, in the latter case, to hold the other factor, or factors, responsible for an explanation of the difficulties in question. It was the object of this paper to suggest another factor in the formation of species, which, although independent of natural selection, was in no way opposed to it, and might be called supplementary to it, and was at the same time capable of explaining the facts, of the inutility of many specific characters, the cross-infertility of allied species, and the non-occurrence of free intercrossing. Very briefly indicated, Mr. Romanes' line of argument is as follows:—Every generation of every species presents an enormous number of variations, of which only the ones that happen to be useful are preserved by natural selection. The useless variations are allowed to die out immediately by intercrossing.
Consequently, if intercrossing be prevented, there is no reason why unuseful variations should not be perpetuated by heredity quite as much as useful ones when under the nursing influence of natural selection. Thus, if from any cause, a section of a species is prevented from intercrossing with the rest of its parent form, it is to be expected that new varieties—for the most part of a trivial and unuseful kind—should arise within that section, and in time pass into new species. This supposition is borne out by the nature of the flora and fauna of oceanic islands, which are particularly rich in peculiar species, and where intercrossing was, of course, prevented with the original parent forms by the action of the geographical boundaries.
However, closely allied species are not always, or even generally, separated by geographical boundaries, and the cross-infertility remains to be explained. The cardinal feature of Mr. Romanes' theory is that the initial step in the origin of species is the arising of this infertility as an independent variation, by which, free intercrossing with the parent form on a common area is prevented, and specific differentiation rendered possible. Innumerable varieties are known to occur which do not pass into distinct species, the reason being that this initial variation, that is, incipient infertility whereby the swamping effects of intercrossing might be obviated, was lacking, and the variations became re-absorbed. That is, given any degree of sterility towards the parental form which does not extend to the varietal form, then a new species must take its origin. Without the bar of sterility, in Mr. Romanes' opinion, free intercrossing must render the formation of species impossible. Mutual sterility is thus the cause, not the result, of specific differentiation. As regards the occurrence of this initial variation, the reproductive system is known to be highly variable, its variability taking the form either of increased fertility, or of sterility in all degrees, and depending on either extrinsic causes (changes of food, climate, &c.), or on an intrinsic cause arising in the system itself.
From the nature of this additional factor at work in the formation of species, Mr. Romanes called his theory 'physiological selection.'
Physiological selection is conceived of as co-operating with natural selection, the former allowing the latter to act by interposing its law of sterility, with the result that the secondary specific characters may be either adaptive or non-adaptive in character.