portant facts concerning the nationality of the persons. It would appear that the Evans's and Jones's were of Welsh descent, and those whose names bear the prefix Mac of Scotch descent. With the nationality would be more or less strictly correlated many peculiarities of physical constitution, language, habits, or mental character. In other cases I have been interested in noticing the empirical inferences which are displayed in the most apparently arbitrary arrangements. If a large register of the names of ships be examined it will often be found that a number of ships bearing the same name were built about the same time, a correlation due to the occurrence of some striking incident shortly previous to the building of the ships. The age of ships or other structures is usually closely correlated with their general form, nature of materials, &c.

It is impossible to examine the details of some of the most apparently artificial systems of classification of plants, without finding that many of the classes are natural in character. Thus in Tournefort's arrangement, depending almost entirely on the formation of the corolla, we find the natural orders of the Labiata, Cruciferæ, Rosacea, Umbelliferæ, Liliacea, and Papilionaceæ, recognise in his 4th, 5th, 6th, 7th, 9th, and 10th classes. Many of the classes in Linnæus' celebrated sexual system also approximate to natural classes.

Correlation of Properties.

Habits and usages of language are always apt to lead us into the error of imagining that when we employ different words we mean different things. In introducing the subject of classification nominally I was careful to draw the reader's attention to the fact that all reasoning and all operations of scientific method really involve classification, though we are accustomed to use the name

in some cases and not in others. Now the name correlation requires to be used with the same qualification. Things are correlated (con, relata) when they are so related or bound to each other that where one is the other is, and where one is not the other is not. Throughout this work we have then been dealing with correlations. In geometry the occurrence of three equal angles in a triangle is correlated with the existence of three equal sides; in physics gravity is correlated with inertia; in botany exogenous growth is correlated with the possession of two cotyledons, or the production of flowers with that of spiral vessels. But it is in the classificatory sciences especially that the word correlation has been employed.

We find it stated that in the class Mammalia the possession of two occipital condyles, with a well-ossified basi-occipital, is correlated with the possession of mandibles, each ramus of which is composed of a single piece of bone, articulated with the squamosal element of the skull, and also with the possession of mammæ and nonnucleated red blood-corpuscles. Professor Huxley remarks d that this statement of the character of the class mammalia is something more than an arbitrary definition; it is a statement of a law of correlation or co-existence of animal structures, from which most important conclusions are deducible. It involves a generalization to the effect that in nature the structures mentioned are always found associated together. This simply amounts to saying that the formation of the class mammalia involves an act of inductive discovery, and results in the establishment of certain empirical laws of nature. Professor Huxley has excellently expressed the mode in which discoveries of this kind enable naturalists to make deductions or predictions

d Lectures on the Elements of Comparative Anatomy, and on the Classification of Animals,' 1864, p. 3.

with considerable confidence, but he has also pointed out that such inferences are likely from time to time to prove mistaken. I will quote his own words:

If a fragmentary fossil be discovered, consisting of no more than a ramus of a mandible, and that part of the skull with which it articulated, a knowledge of this law may enable the palæontologist to affirm, with great confidence, that the animal of which it formed a part suckled its young, and had non-nucleated red blood-corpuscles; and to predict that should the back part of that skull be discovered, it will exhibit two occipital condyles and a well-ossified basi-occipital bone.

'Deductions of this kind, such as that made by Cuvier in the famous case of the fossil opossum of Montmartre, have often been verified, and are well calculated to impress the vulgar imagination; so that they have taken rank as the triumphs of the anatomist. But it should carefully be borne in mind, that, like all merely empirical laws, which rest upon a comparatively narrow observational basis, the reasoning from them may at any time break down. If Cuvier, for example, had had to do with a fossil Thylacinus instead of a fossil Opossum, he would not have found the marsupial bones, though the inflected angle of the jaw would have been obvious enough. And so, though, practically, any one who met with a characteristically mammalian jaw would be justified in expecting to find the characteristically mammalian occiput associated with it; yet, he would be a bold man indeed, who should strictly assert the belief which is implied in this expectation, viz., that at no period of the world's history did animals exist which combined a mammalian occiput with a reptilian jaw, or vice versa.'

One of the most distinct and remarkable instances of correlation in the animal world is that which occurs in ruminating animals, and which could not be better stated

than in the following extract from the classical work of Cuviere:

'I doubt if any one would have divined, if untaught by observation, that all ruminants have the foot cleft, and that they alone have it. I doubt if any one would have divined that there are frontal horns only in this class that those among them which have sharp canines for the most part lack horns.

'However, since these relations are constant, they must have some sufficient cause; but since we are ignorant of it, we must make good the defect of the theory by means. of observation it enables us to establish empirical laws which become almost as certain as rational laws when they rest on sufficiently repeated observations; so that now whoso sees merely the print of a cleft foot may conclude that the animal which left this impression ruminated, and this conclusion is as certain as any other in physics or morals. This footprint alone, then, yields to him who observes it, the form of the teeth, the form of the jaws, the form of the vertebræ, the form of all the bones of the legs, of the thighs, of the shoulders, and of the pelvis of the animal which has passed by: it is a surer mark than all those of Zadig.'

We meet with a good instance of the purely empirical correlation of circumstances when we classify the planets of the solar system according to their densities or periods of axial rotation'. If we examine a table specifying the usual astronomical numbers of the solar system, we find that four planets resemble each other very closely in the period of axial rotation, and the same four planets are all found to have high densities, thus:

• 'Ossemens Fossiles,' 4th edit. vol. i. p. 164. Quoted by Huxley, 'Lectures,' &c., p. 5.

f Chambers, Descriptive Astronomy,' 1st edit. p. 23.

Forming a similar table for the other chief planets, it is

is

It will of course be observed that in neither group the equality of the rotational period or of the density more than rudely approximate, nevertheless the difference of the numbers in the first and second group is so very marked, the periods of the first being at least double and the densities four or five times those of the second, that the coincidence cannot be attributed to accident. The reader will also notice that the first group consists of the planets nearest to the sun, that with the exception of the earth none of them possess satellites, and that they are all comparatively small; the second group are furthest from the sun, and all of them possess several satellites, and are comparatively great. Therefore, with but slight exception, the following correlations hold true :

These coincidences certainly point with much probability to a difference in the conditions of origin of the two groups, but no further explanation of the matter is yet possible.

The classification of comets by Mr. Hind and Mr. A. S. Davis according to their periods tends to establish the conclusion that distinct groups of comets have been