they seem sometimes to have this value. But this cannot be the whole truth. Apart from the just objections that thorns are often prevalent in countries where there are few grazing animals and that they do not appear in the early stages when they are most needed, experiment has shown that many thorns arise in response to poor nutrition.' Thorns are the natural outcrops of a kind of constitution suited to dry countries.

This idea was familiar enough to Darwin, as we see from the emphasis which he laid on instances of "correlated variability." In this connection, Sir Ray Lankester observed, at the Cambridge Centenary Celebrations : "In my opinion he has thus furnished the key to the explanation of what are called useless specific characters and of incipient organs. That key consists in the fact that a general physiological property, or character of utility, is often selected and perpetuated which carries with it distinct, even remote, correlated growths and peculiarities obvious to our eyes, yet

1 See Coulter, op. cit. (1909), and Geddes, "Proc. Brit. Assoc." (1889.) Prof. Coulter points to significant facts like the following: the nettle can get on quite well without its stinging hairs; many seeds, especially in arid regions, develop a testa so hard that it interferes with the breaking through of the embryo—which looks like over-adaptation"; further investigation has played havoc with the pretty story of the extra-floral nectaries attracting a body-guard of harmless ants. It is probable that in these and a hundred other cases our task is rather that of discovering the physiological and embryological significance of the structures in question, than that of searching diligently for a utilitarian justification which does not exist. A familiar example may be found in our finger-prints, which illustrate discontinuity in evolution-the apparently abrupt origin of new patterns; but, as we have no warrant for supposing that natural selection operates in any way in this case, we must suppose that these patterns are the expressions of internal growthconditions.

having no functional value. At a later stage in the history of such a form these correlated growths may acquire value and become the subject of selection." 1

(d) Among palæontologists, too, there are some, like Prof. H. F. Osborn, who make out a strong case for the origin of new characters by definite progressive variation, and "not by the selection of the fit from the fortuitous.' In other words, many paleontologists claim that indefinite variations off the main line are absent, so far as the rock-record tells.

"The law of gradual appearance or origin of many new characters in definite or determinate directions from the very beginning I regard as the grandest contribution which palæontology has made to evolution." 2 We must attach great importance to this expression of opinion, for it is shared by many who, like Prof. Osborn, have given their life to studying the actual history; but it must be borne in mind that highly specialised types, like Ammonites and mammals, may be like well-pruned trees-they may have been selected through long periods into lines of determinate variation. The power of divergent idiosyncrasy may have been pruned out of them.

After referring to the work of Waagen on Ammonites and his own work on mammals (e.g. their teeth), Prof. H. F. Osborn says: "The law of gradual change in certain determinate, definite, and, at least in some cases, adaptive directions, through very long periods of time, and the absence

1 Nature (July 1, 1909), p. 10.

"Darwin and Palæontology," by H. F. Osborn, in “ Years of Darwinism" (1909).

Fifty

of chance and non-direction in the origin of a large number of adaptive and other new characters, is the common working principle both in Vertebrate and Invertebrate paleontology."

1

(e) Another change of view-rank heresy to those of the straiter sect of Darwinians-is seen in the writings of not a few naturalists who do not feel themselves bound to find a use for everything. There are many apparently trivial characters for which careful investigation has discovered very definite and unexpected utility-Weismann gives, as an example, the beautiful microscopic anchors and discs of lime found in the skin of the burrowing, worm-like Holothurians known as Synaptids; but, on the other hand, the tyranny of an extreme zoological utilitarianism may become absurd. When the wind blows the long, sharp-pointed leaf of the sand-binding bent-grass it often makes a perfect circle on the sand, but there is no significance in this. Nor is there in the beautiful ripplemarks on the sand or in the frost-flowers in the window. It seems likely that there are many such things in living creatures-registrations of orderly functional rhythms, the ripple-marks of periodic growth. The cross barring of a feather may simply express diurnal variations in bloodpressure when the feather was developing. That it may come to be useful is another matter.

SEXUAL SELECTION.-As a corollary to his theory of natural selection, Darwin expounded a theory of sexual selection, in which he interpreted some of the secondary peculiarities of the sexes as the outcome of selective processes involved in the combats of rival suitors and in the choice

1 Op. cit. (1909).

exercised by the coy females. All sorts of masculine weapons, such as antlers; all sort of decorations, such as brilliant plumage; all sorts of excitants, such as love-calls and fragrance, may be interpreted in terms of the sexual selection which seems to occur in many cases, especially where there are more males than females, or where polygamy occurs. The whole matter is difficult, perhaps more difficult than Darwin thought, and there is great difference of opinion in regard to it. Wallace did not see his way to believing at all in the action of female choice; Weismann was whole-heartedly with Darwin. It must suffice to refer to a few of the post-Darwinian contributions to this fascinating subject.

(a) The combats of rival males are often very fierce. The younger or weaker candidates may be killed, or expelled, or left unmated. In such cases there seems little reason to doubt the discriminateness of the elimination. In some curious cases, as in spiders, the tournaments are prolonged, but the combatants do not seem to hurt one another; and it is possible that the significance of the jousting is to excite the females, who sometimes stand by, as it were interested spectators. In some other cases, e.g. among Lamellicorn beetles and Bearded Monkeys, there seems to be more bluffing than fighting, for precedence is given to the candidate of most imposing appearance.

(b) In regard to those masculine characters which indubitably attract the female and probably serve to excite her and to overcome her coyness, Wallace consistently maintained that there was very little clear evidence that the female chooses a partner out of a number of suitors. At the same

The males are to the left, the females to the right.

A and a. Male and female Hercules Beetles (Dynastes).

B and b. Male and female spiders, Nephila nigra (after Vinson).

C and c. Male and female crested newts, Triton (or Molge) cristatus. The male has