Species and Varieties, Their Origin by Mutation | Page 6

the proposed theories to a severe criticism both on the ground of facts and on that of their innate possibility and logical value. He decides in favor of the mutation theory. His arguments are incisive and complete and wholly adapted to the comprehension of all intelligent readers, so that his book relieves me entirely of the necessity of discussing these general questions, as it could not be done in a better or in a clearer way.
I intend to give a review of the facts obtained from plants which go to prove the assertion, that species and varieties have originated by mutation, and are, at present, not known to originate in any other way. This review consists of two parts. One is a critical survey of the facts of agricultural and horticultural breeding, as they have accumulated since the time of Darwin. This body of evidence is to be combined with some corresponding experiments [10] concerning the real nature of species in the wild state. The other part rests on my own observations and experiments, made in the botanical garden of the University of Amsterdam.
For many years past I have tried to elucidate the hereditary conditions of species and varieties, and the occasional occurrence of mutations, that suddenly produce new forms.
The present discussion has a double purpose. On one side it will give the justification of the theory of mutations, as derived from the facts now at hand. On the other hand it will point out the deficiencies of available evidence, and indicate the ways by which the lacunae may gradually be filled. Experimental work on heredity does not require vast installments or costly laboratory equipment. It demands chiefly assiduity and exactitude. Any one who has these two qualities, and who has a small garden at his disposal is requested to take part in this line of investigation.
In order to observe directly the birth of new forms it is necessary, in the first place, to be fully clear concerning the question as to what forms are to be expected to arise from others, and before proceeding to a demonstration of the origin of species, it is pertinent to raise the question as to what constitutes a species.
Species is a word, which always has had a [11] double meaning. One is the systematic species, which is the unit of our system. But these units are by no means indivisible. Long ago Linnaeus knew them to be compound in a great number of instances, and increasing knowledge has shown that the same rule prevails in other instances. Today the vast majority of the old systematic species are known to consist of minor units. These minor entities are called varieties in systematic works. However, there are many objections to this usage. First, the term variety is applied in horticulture and agriculture to things so widely divergent as to convey no clear idea at all. Secondly, the subdivisions of species are by no means all of the same nature, and the systematic varieties include units the real value of which is widely different in different cases. Some of these varieties are in reality as good as species, and have been "elevated," as it is called by some writers, to this rank. This conception of the elementary species would be quite justifiable, and would at once get rid of all difficulties, were it not for one practical obstacle. The number of the species in all genera would be doubled and tripled, and as these numbers are already cumbersome in many cases, the distinction of the native species of any given country would lose most of its charm and interest.
[12] In order to meet this difficulty we must recognize two sorts of species. The systematic species are the practical units of the systematists and florists, and all friends of wild nature should do their utmost to preserve them as Linnaeus has proposed them. These units however, are not really existing entities; they have as little claim to be regarded as such as genera and families. The real units are the elementary species; their limits often apparently overlap and can only in rare cases be determined on the sole ground of field observations. Pedigree-culture is the method required and any form which remains constant and distinct from its allies in the garden is to be considered as an elementary species.
In the following lectures we shall consider this point at length, to show the compound nature of systematic species in wild and in cultivated plants. In both cases, the principle is becoming of great importance, and many papers published recently indicate its almost universal acceptation.
Among the systematic subdivisions of species, not all have the same claim to the title of elementary species. In the first place the cases in which the differences may occur between parts of the same