reproductive elements become distinct from the parts which develop into the other organs of the body. In this respect Weissmann's hypothesis has an additional logical support. It has provoked earnest discussion, and naturalists have taken sides on the subject. Although the germ plasm is itself an assumption, and the hypothesis owes its existence to the known facts of karyokinesis, it has led to further investigations in some directions with fruitful results. Darwin's hypothesis opened the question under the new light that he had shed upon nature, and the more recent hypotheses, like his own, owe their existence to the "impelling force" of his general theories of descent and natural selection.

IX.

DEDUCTION. — EXPLANATION OF KNOWN FACTS.

- IMPORTANCE OF THEORY TO GOOD

DAR

OBSERVATION.

ARWIN fell upon the true cause of the modification of species so early, that the greater number of his special investigations took on a deductive cast.1 His reflections upon the known facts and principles of biology took the form of efforts to explain them as deductions from his theory; and many of his new discoveries were foreseen as consequences of

it. Hence the inductive process does not play so important a part in his work as does the inverse process of deduction.

It will not be necessary to dwell long upon his success in giving the proper theoretical explanations to already known facts and empirical laws. It had required centuries of painstaking research and numberless efforts at classification before anything like a natural classification was

1 See the Chapter on the Logical History of the Principle of Natural Selection, post, p. 212.

reached. When, at the beginning of this century, it was finally approached, and the natural affinities of plants and animals were brought out by it, the doctrine of descent was inevitable; and it came. When Darwin had once become impressed with its truth, and had found the cause of modification, it was first of all necessary to show that the great bodies of known facts harmonized with his doctrines. The facts of distribution, palæontology, embryology, rudimentary organs, etc., were all reduced. Each set of facts presented its own difficulties.

Up to the present century it was regarded as an axiom in taxonomy that the structures of most importance to the animals possessing them must be of most importance for the purposes of classification. It is worth while to note that this was accepted as self-evident, as being beyond the necessity of proof. Systematists were approaching the "natural arrangement," and De Candolle discovered empirically the rule that there is usually an inverse ratio between the taxonomic and the functional value of a structure; but he could suggest no reason for the paradox. Darwin's theory furnished the philosophical explanation.1 The

1 Origin of Species, pp. 362-373. Romanes, Darwin and After Darwin, pp. 34-37.

organs of the highest functional value are under the constant and pressing necessity of changing with changes in the environment; while those of least functional value remain undisturbed, and pass, little or not at all modified, from generation to generation. The same explanation holds for the rule concerning the importance of "aggregates of unimportant characters" in determining the affinities of animals. and plants.

In some parts of the natural system, there are what are called "chains of affinities." A group, instead of being broken up into well separated sub-groups, consists of a chain in which the adjacent parts are closely related, but the more distant parts have comparatively few points in common. It is impossible to break up the group without violating the affinities of adjacent parts, and it is difficult to define it in such a way as to include the extremes. The Crustacea furnish an example. What was a special difficulty under the old views of classification is explained under the doctrine of descent. 1

One of the most important principles that had been established empirically was the treelike arrangement of species and higher groups

1 Origin of Species, p. 368.

in the natural classification. The recognition of the principle came only after centuries of efforts at classification; and after it was discovered, no reason could be given for it. Although so helpful and striking, it remained a profound enigma, for there was nothing in the nature of the things classified that required this peculiarly complex arrangement rather than one of several conceivable simpler ones. Darwin's explanation of this principle under his theory illustrates not only the explanation of empirical laws, but the difficulty of doing what, after it is done, seems very simple.

It was not until after he had been at work upon the principle of natural selection for many years, that the true explanation of this law under his principle occurred to him. He said, "I suppose I must be a very slow thinker, for you would be surprised at the number of years it took me to see clearly what some of the problems were which had to be solved; such as the necessity of the principle of divergence of character, the extinction of intermediate varieties, on a continuous area, with graduated conditions," etc. After describing his earlier sketches of his theory he said, "At that time I overlooked one problem of great im1 Life and Letters, Vol. I. pp. 68, 524.