Reginald Crundall Punnett

“ As Darwin long ago realised, any theory of evolution must be based upon the facts of heredity and variation. Evolution only comes about through the survival of certain variations and the elimination of others. But to be of any moment in evolutionary change a variation must be inherited. And to be inherited it must be represented in the gametes. This, as we have seen, is the case for those variations which we have termed mutations. ”

“ If there is any difference in keenness of perception we shall probably not be far wrong in presuming that the advantage rests with the bird. After all if there is any truth in the theory of mimicry the bird has to depend largely upon its keenness of sight in making its living, at any rate if that living is to be a palatable one. If natural selection can bring about these close resemblances among butterflies it must certainly be supposed to be capable of bringing the bird's powers of vision to a high pitch of excellence. ”

“ Moreover when the perfect thing is once elaborated natural selection will keep it perfect by discouraging any tendency to vary from perfection.
Upon this view, of which the most distinguished protagonist was Weismann, natural selection is the sole arbiter of animal and plant form. Through it and it alone the world has come to be what it is. To it must be ascribed all righteousness, for it alone is the maker. Such in its extreme form is the modern development of Darwin's great contribution to philosophy. ”

“ When we speak of progress we generally mean that the hoard has been improved, and is of more service to man in his attempts to control his surroundings. Sometimes this hoarded knowledge is spoken of as the inheritance which a generation receives from those who have gone before. This is misleading. The handing on of such knowledge has nothing more to do with heredity in the biological sense than has the handing on from parent to offspring of a picture, or a title, or a pair of boots. All these things are but the transfer from zygote to zygote of something extrinsic to the species. ”

“ But the majority of workers have taken natural selection for granted and then interested themselves in shewing how the characters studied by them might be of use. Probably there is no structure or habit for which it is impossible to devise some use [2] , and the pursuit has doubtless provided many of its devotees with a pleasurable and often fascinating exercise of the imagination. So it has come about that the facts instead of being used as a test of the credibility of natural selection, serve merely to emphasise the pæan of praise with which such exercises usually conclude. ”

“ Those who have examined long series of these cases of resemblance among butterflies find it hard to believe that there is not some connection between them apart from climatic influence. One feels that they are too numerous and too striking to be all explained away as mere coincidences engendered by like conditions. Nor is it improbable that natural selection in the form of the discriminating enemy may have played a part in connection with them, though a different one from that advocated on the current theory of mimicry. ”

“ We have now reached a point at which it is possible to formulate a definite conception of the living organism. A plant or animal is a living entity whose properties may in large measure be expressed in terms of unit-characters, and it is the possession of a greater or lesser number of such unit-characters renders it possible for us to draw sharp distinctions between one individual and another. These unit-characters are represented by definite factors in the gamete which in the process of heredity behave as indivisible entities, and are distributed according to a definite scheme. ”

“ On the mainland, however, the enemies of Pseudacraea are well acquainted with the Planema models which are here common, and discriminate against individuals which are not close mimics of the Planemas. The result is that on the mainland transitional forms are scarcer than on the island. Natural selection maintains a high standard for the mimetic likeness on the mainland owing to the abundance of the model; but when the model is scarce the likeness ceases to be kept up to the mark strictly, and tends to become lost owing to the appearance of fresh variations which are no longer weeded out. ”

“ Either natural selection, from the point of view of mimicry, is non-existent for this species in Ceylon, or else it is so slight as to be unable in half a century to produce an appreciable diminution in the proportion of non-mimetic females. For even if the mimetic resemblance brings about but the survival of one additional protected form in 100 as compared with the unprotected, this means a marked diminution in the proportion of M females in 50 years—a diminution such as there are no grounds for supposing to have taken place. ”

“ A new impression cannot fail to be welcomed by those who already have it as well as those who have it not.... Mendel's Principles of Heredity is already a classic. It marks a position of stability towards which previous work is now seen to have logically converged, and from which new and active research is to-day no less logically diverging. The various waves of biological thought are constantly intersecting, mingling, and passing on with altered rhythm, but it rarely happens that so many meet together at a nodal point as during the last decade.... ”

“ Unless the bird were keener sighted than the man the A form would be more likely to be taken in mistake for its unprotected relative than avoided for its resemblance to the presumably unpalatable model. On the other hand, if the bird were sufficiently keen sighted never to confuse the A female with the male form its sight would be too keen to be imposed upon by such resemblance as exists between the A female and P. aristolochiae.
These, however, are not the only criticisms of the theory of mimicry which the study of this species forces upon us. ”

“ The advent of natural selection threw a new light upon adaptation and the appearance of design in the world. In such books as those on The Fertilization of Orchids and The Forms of Flowers Darwin sought to shew that many curious and elaborate structures which had long puzzled the botanist were of service to the plant, and might therefore have arisen through the agency of natural selection. ”

“ There are writers on these questions of mimicry who adopt a view more or less intermediate between those just discussed. They regard the resemblance as having arisen in the first place as a sport of some magnitude on the part of the mimic, rendering it sufficiently like the model to cause some confusion between the two. A rough-hewn resemblance is first brought about by a process of mutation. Natural selection is in this way given something to work on, and forthwith proceeds to polish up the resemblance until it becomes exceedingly close. ”

“ As a general rule, the cross between a male and a female results in the production of the two sexes in approximately equal numbers. The cross between a heterozygous dominant and a recessive also leads to equal numbers of recessives and of heterozygous dominants. Is it not, therefore, possible that one of the sexes is heterozygous for a factor which is lacking in the other, and that the presence or absence of this factor determines the sex of the zygote? The results of some recent experiments would appear to justify this interpretation, at any rate in particular cases. ”

“ CHAPTER II MIMICRY—BATESIAN AND MÜLLERIAN
Mimicry is a special branch of the study of adaptation. The term has sometimes been used loosely to include cases where an animal, most frequently an insect, bears a strong and often most remarkable resemblance to some feature of its inanimate surroundings. Many butterflies with wings closed are wonderfully like dead leaves; certain spiders when at rest on a leaf look exactly like bird-droppings ”

“ If birds regarded white butterflies as edible it is difficult to suppose that they would be checked in their attacks by a trifling patch of colour while the main ground of the insect was still white. But unless they avoided those with the small colour patch there would be an end of natural selection in so far as the patch was concerned, and it would have no opportunity of developing further through the operation of that factor. This is the difficulty of the initial variation which has been clearly recognised by most of the best known supporters of the theory of mimicry. ”

“ Then in 1859 came the Origin of Species, in which Darwin presented to the world a consistent theory to account for the manner in which one species might have arisen from another by a process of gradual evolution. Briefly put, that theory was as follows: In any species of plant or animal the reproductive capacity tends to outrun the available food supply, and the resulting competition leads to an inevitable struggle for existence. Of all the individuals born, only a portion, and that often a very small one, can survive to produce offspring. ”

“ The colour of an animal or of a flower, the shape of a seed, or the pattern on the wings of a moth are all zygotic properties, and all capable of direct estimation. It is otherwise with the properties of gametes. While the difference between a black and a white fowl is sufficiently obvious, no one by inspection can tell the difference between the egg that will hatch into a black and that which will hatch into a white. Nor from a mass of pollen grains can any one to-day pick out those that will produce white from those that will produce coloured flowers. ”