Mammals transported into colder countries, or subjected to continued exposure, grow a thicker coat; and the same kind of tree which in a sheltered valley is tall, large-leaved, and soft-wooded, assumes a very different aspect, although perhaps growing into a healthy specimen, when planted on a wind-exposed hill.
There is no room, or, rather, no time, to apply to these cases the principle of many variations or the long-continued accumulation of infinitely small changes. The thing is to be done quickly, or not at all. Nor can we explain the mending of a wound, which implies an activity of countless cells, simply as a case of, or similar to, the reproduction of a lost part; against such an explanation militates the almost absolute unlikelihood of that precise injury having happened before to any of the creature's ancestors.
Still, I think we are brought near the solution of the mystery by such considerations. We see no difficulty in the regeneration of a few cells, or in the making good of the disturbance suffered by one of the most simple organisms; but we become suspicious when we see that countless cells, not of one kind, but of the most varied tissues and parts of the body, make common cause in remedying a defect in a serviceable way.
We must assume that since the beginning of life organisms have been subjected to countless insults. We can scarcely speak of a wound in an Amæba; but these insults have always been made good, and whenever this was not the case, that particular organism came to an end. As these organisms developed into more complicated ones, the possible insults became more serious, more complicated; and the organisms took adaptive measures so as to be superior to them. This action, I have no hesitation in declaring, became by heredity a habit. The whole creature became so thoroughly 'imbued' (for want of a better word) with the finding of ways and means for meeting sudden, serious conditions, that it now acts directly, and produces by a short-cut, with the least amount of time and with the smallest possible waste of material, that which meets the occasion, thereby saving the life of the individual and that of the race. This we cannot but call reasonable and to the purpose, although it is all carried out by causæ efficientes without there being any causæ finales.
GEOLOGICAL TIME AND EVOLUTION.
One million years is a stretch of time beyond our conception. We can arrive at a more or less adequate understanding of what a million individuals or concrete things means. Several Continental nations can put more than a million men into the field. We can gaze at a building which contains as many bricks; and we know that our own body is composed of millions of millions of cells. No such help applies to time, because that itself is an entirely relative, abstract conception. We can imagine what one hundred years are like—a span of time seemingly short to the hale and hearty octogenarian, enormous to the child, totally inapplicable to certain animals whose whole life is crowded into one single day.
Astronomers have long ceased to reckon distances by miles or any other understandable unit. They express the distances between us and the stars and nebulæ by 'years of light.' Try to imagine a unit of length equal to that which is passed through by light (186,000 miles per second) in one year. Not so very long ago the enormous distances resulting from astronomical calculations were looked upon as the most serious objection to the correctness of the astronomers' views as to the distances which separate our globe from the nearest fixed stars. We have not yet accustomed ourselves to reckoning time by some similar broadly-conceived standard—say æons of so many thousand years each.
Unfortunately, we possess no data whatever for calculating the age of the successive geological strata. Thanks to Lyell, the theory of violent universal cataclysms has been done away with. It is more probable that the same agencies have acted which are now changing the aspect of the globe; and these changes are slow, as far as we know them—at least, as far as the formation of sedimentary strata is concerned, and these alone we have to deal with. Various calculations have been made, based upon the denudation of the mountains, the filling up of the valleys by the débris, the formation of deltas, etc. The results give enormous stretches of time, but all of them unsatisfactory, because the methods are so very local in their application.