66 generations penetrated deeper and deeper into the cave. In such cases blindness is often compensated by an improvement in the organs of feeling, such as antennæ. "It may be metaphorically said that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of Time has marked the lapse of ages, and then so imperfect is our view into long-past geological ages, that we see only that the forms of life are now different from what they formerly were." (Origin of Species, ch. iv.) Thus Darwin enunciated the doctrine of evolution with regard to the organic life of the world. The lines of investigation that he followed and the conclusions he reached have been stated at some length, because it may truly be said that he established a new mode of thinking about the universe, though it is quite possible that he attributed too much to natural selection in the production of new forms, and too little to the experimental tendency which life appears to possess. At any rate, life proceeds by evolution, and this is equally true as regards the inorganic world. Nor do we believe that evolution is confined to our planet, the earth. We believe that it prevails throughout the universe, and that the stars and the other planets are in evolution likewise. T CHAPTER VI THE EVOLUTION OF PLANTS HE plants which clothe the surface of the earth at the present time form one stage in the whole evolution of plant life. That life, like everything else in the universe, will continue subject to change, and what the next phase will be we cannot foresee. We can, however, discover something about plant evolution in the past. We know that our cultivated plants are descended from wild ones, and that their differences are due to the choice by man of the conditions under which they exist, and of the individuals from which new plants are raised. It is reasonable, then, to infer that our wild plants have also evolved from older types which have gradually been modified, mainly as a result of natural selection and change of environment. We can get direct evidence as to this from ancient plants themselves, embedded as fossils in the rocks. The record is not complete; it is less so even than that of animals; but where a plant has been fossilized its cell structure has often been preserved with wonderful completeness. We can also study the different types of plants that exist to-day-the single-celled plants, such as protococcus, the seaweeds, the lichens, and ferns, the conebearing trees, and the flowering plants which are at present the dominant family of the vegetable kingdom. We shall find that these represent what we believe to have been the main stages of plant evolution; and if we can discover links or possible lines of connexion between these very unlike types we shall have indirect but fairly reliable evidence of the descent and relationship of plants in the past. The result of much study both of fossil and existing plants during recent years is that there no longer appear to be impassable gulfs set between the great families that dominated the plant world at different epochs; much, however, remains to be done before the exact relationships between them and between the smaller plant groups can be looked upon as established beyond a doubt. For example, the apparently sudden appearance of flowering plants during the formation. of the chalk rocks is a mystery which the study of evolution has not yet solved. The development of plant and animal life is undoubtedly closely related, and just before the flowering plants the higher insects, bees, wasps, moths, and butterflies had appeared. These two facts probably have a bearing upon one another, and in this as in other cases we must not expect to find in change of environment alone an explanation of the differences which exist between the great plant families of different eras. The rocks of the coal measures, for instance, were being laid down at a time when the climate and other physical conditions of the earth must have been very much what they are in some parts to-day. Yet the coal measures, rich as they are in fossil plants, contain not a single fossil of the flowering plants that appear later in the chalk rocks. Quite apart from the influence of environment there appears to be in nature a tendency to produce spontaneously new developments in the individuals of a species-what in cultivated plants gardeners term sports--and these new lines of development may have the most vital influence on the species. For if they benefit the individual showing them, that individual and its descendants, if they inherit them, will have a better chance in the struggle for existence, and the other members of the species will be at a disadvantage. In such developments environment has perhaps no more influence, says Darwin, "than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames." Again, evolution must not be thought of as invariably a progression from a lower to a higher organism, from a simple to a more complex. It is true that in each division of the earth's history since life began the vegetable kingdom increases both in variety and perfection. Yet, however simple an organism, it will survive if it is perfectly adapted to its surroundings, and a complex organism sometimes simplifies itself if by doing so it attains this end. Thus some flowering plants which have taken to fresh water have simplified their structure to compete with native water plants, and have lost all likeness to flowering plants because flowers are not suited to water life. For this reason when we examine simple organisms we cannot always be certain whether they represent a survival of a primitive type through perfect adaptation or a degeneration of a higher form, and this doubt exists concerning bacteria, the very simplest organisms, since they consist of a tiny single cell, possibly not even possessing a nucleus. Still, since perfect adaptation ensures survival, there is no reason to doubt that some of the simplest plant organisms of to-day may be the living representatives of some of the first plants evolved from the cell that was neither animal nor plant by the formation of the green colouring matter called chlorophyll. As has been stated in an earlier chapter, life was first born probably on the warm, muddy shores of primeval lakes and seas, and here too and on the waters themselves would lie and float the green cells that were the first plants. Protococcus, our simplest living plant, is a waterdweller still. It consists of a single cell containing protoplasm, a nucleus, and a green colouring body. It reproduces itself by dividing into four parts, similar in structure to the original cell, which remain within the wall for a time before they start an independent existence of their own. Simple as such an organism is, it is capable of development, and the ancestors of the highest plants probably started in as lowly a way. If the four cell wall colouring matter nucleus PROTOCOCCUS daughter-cells fail to complete their separation from the mother-cell, but surround themselves with walls and proceed to divide again, we have the beginning of a cellular structure. This may become more and more complex by the modification of cells for the performance of work important to the plant as a whole, and by their building up into various tissues. In this way the higher plants must have evolved. As has been mentioned, the record of their evolution contained in the rocks is very imperfect. It is true that we have fossil plants millions of years old, the structure of which has been so perfectly preserved that it can be studied under the microscope in almost as much detail as that of a freshly gathered specimen. But of the plants that flourished during vast stretches of time after the appearance of the first green cells we know practically nothing. We should not expect the soft tissues of the earliest plants to survive. Even if |