Species and Varieties, Their Origin by Mutation | Page 7

individual are to be excluded. Dividing an alpine plant into two halves and [13] planting one in a garden, varietal differences at once arise and are often designated in systematic works under different varietal names. Secondly all individual differences which are of a fluctuating nature are to be combined into a group. But with these we shall deal later.
Apart from these minor points the subdivisions of the systematic species exhibit two widely different features. I will now try to make this clear in a few words, but will return in another lecture to a fuller discussion of this most interesting contrast.
Linnaeus himself knew that in some cases all subdivisions of a species are of equal rank, together constituting the group called species. No one of them outranks the others; it is not a species with varieties, but a group, consisting only of varieties. A closer inquiry into the cases treated in this manner by the great master of systematic science, shows that here his varieties were exactly what we now call elementary species.
In other cases the varieties are of a derivative nature. The species constitutes a type that is pure in a race which ordinarily is still growing somewhere, though in some cases it may have died out. From this type the varieties are derived, and the way of this derivation is usually quite manifest to the botanist. It is ordinarily [14] by the disappearance of some superficial character that a variety is distinguished from its species, as by the lack of color in the flowers, of hairs on stems and foliage, of the spines and thorns, &c. Such varieties are, strictly speaking, not to be treated in the same way as elementary species, though they often are. We shall designate them by the term of "retrograde varieties," which clearly indicates the nature of their relationship to the species from which they are assumed to have sprung. In order to lay more stress on the contrast between elementary species and retrograde varieties, it should be stated at once, that the first are considered to have originated from their parent-form in a progressive way. They have succeeded in attaining something quite new for themselves, while retrograde varieties have only thrown off some peculiarity, previously acquired by their ancestors.
The whole vegetable kingdom exhibits a constant struggle between progression and retrogression. Of course, the great lines of the general pedigree are due to progression, many single steps in this direction leading together to the great superiority of the flowering plants over their cryptogamous ancestors. But progression is nearly always accompanied by retrogression in the principal lines of evolution, [15] as well as in the collateral branches of the genealogical tree. Sometimes it prevails, and the monocotyledons are obviously a reduced branch of the primitive dicotyledons. In orchids and aroids, in grasses and sedges, reduction plays a most important part, leaving its traces on the flowers as well as on the embryo of the seed. Many instances could be given to prove that progression and retrogression are the two main principles of evolution at large. Hence the conclusion, that our analysis must dissect the complicated phenomena of evolution so far as to show the separate functions of these two contrasting principles. Hundreds of steps were needed to evolve the family of the orchids, but the experimenter must take the single steps for the object of his inquiry. He finds that some are progressive and others retrogressive and so his investigation falls under two heads, the origin of progressive characters, and the subsequent loss of the same. Progressive steps are the marks of elementary species, while retrograde varieties are distinguished by apparent losses. They have equal claim to our interest and our study.
As already stated I propose to deal first with the elementary species and afterwards with the retrograde varieties. I shall try to depict them to you in the first place as they are seen in [16] nature and in culture, leaving the question of their origin to a subsequent experimental treatment.
The question of the experimental origin of new species and varieties has to be taken up from two widely separated starting points. This may be inferred from what we have already seen concerning the two opposing theories, derived and isolated from Darwin's original broad conception. One of them considers mutations as the origin of new forms, while the other assumes fluctuations to be the source of all evolution.
As mentioned above, my own experience has led me to accept the first view. Therefore I shall have to show that mutations do yield new and constant forms, while fluctuations are not adequate to do so. Retrograde varieties and elementary species may both be seen to be produced by sudden mutations. Varieties have often been observed to appear at once and quite unexpectedly in horticulture and