These new species are not produced once or in single individuals, but yearly and in large numbers. The whole phenomenon conveys the idea of a close group of mutations, all belonging to one single condition of mutability. Of course this mutable state must have had a beginning, as it must sometime come to an end. It is to be considered as a period within the life-time of the species and probably it is only a small part of it.

The detailed description of this experiment, however, I must delay to a subsequent lecture, but I may be allowed to state, that the discovery of this period of mutability is of a definite theoretical importance. One of the greatest objections to the Darwinian theory of descent arose from the length of time it would require, if all evolution was to be explained on the theory of slow and nearly invisible changes. This difficulty is at once met and fully surmounted by the hypothesis of periodical but sudden and quite noticeable steps. This assumption requires only a limited number of mutative periods, which might well occur within the time allowed by physicists and geologists for the existence of animal and vegetable life on the earth.

[30] Summing up the main points of these introductory remarks, I propose to deal with the subjects mentioned above at some length, devoting to each of them, if possible at least an entire lecture. The decisive facts and discussions upon which the conclusions are based will be given in every case. Likewise I hope to point out the weak places and the lacunae in our present knowledge, and to show the way in which each of you may try to contribute his part towards the advancement of science in this subject. Lastly I shall try to prove that sudden mutation is the normal way in which nature produces new species and new varieties. These mutations are more readily accessible to observation and experiment than the slow and gradual changes surmised by Wallace and his followers, which are entirely beyond our present and future experience.

The theory of mutations is a starting-point for direct investigation, while the general belief in slow changes has held back science from such investigations during half a century.

Coming now to the subdivisions and headings under which my material is to be presented, I propose describing first the real nature of the elementary species and retrograde varieties, both in normal form and in hybridizations. A discussion of other types of varieties, including [31] monstrosities will complete the general plan. The second subdivision will deal with the origin of species and varieties as taught by experiment and observation, treating separately the sudden variations which to my mind do produce new forms, and subsequently the fluctuations which I hold to be not adequate to this purpose.

[32] B. ELEMENTARY SPECIES

LECTURE II

ELEMENTARY SPECIES IN NATURE

What are species? Species are considered as the true units of nature by the vast majority of biologists. They have gained this high rank in our estimation principally through the influence of Linnaeus. They have supplanted the genera which were the accepted units before Linnaeus. They are now to be replaced in their turn, by smaller types, for reasons which do not rest upon comparative studies but upon direct experimental evidence.