An avoidance of the threatened destruction of a species by its adaptation to the new circumstances can only be possible when the changes occur very slowly, and will therefore be more likely to be achieved in the case of physical changes in the conditions of life, such as climatic changes, a change in the mutual relations of land and sea, and so on. But it appears that even climatic changes do not evoke any variation and new adaptation as long as the species can avoid the changes by migrating. The often quoted case of Alpine and Arctic plants proves this at any rate, that those species which inhabited the plateaus and highlands of Europe did not all vary to suit the change when a warmer climate prevailed, but that in part at least they followed the climate to which they were already adapted, that is, that they migrated towards the north on the one hand and higher up the Alps on the other. It cannot be denied that many of the insects and plants did adapt themselves at that time to the warmer climate, and became the modern species which now inhabit the plains, for many related species occur on the Alps and in the plains, but apparently many others simply made their escape from a climate which no longer suited their requirements. Thus, as far as I am aware, there is no species of Primula in South or Central Germany which could be derived from the beautiful red Primula farinosa of the Alps, but this species occurs also upon the old glacier-soil at the northern base of the Alps, and in similar soil again in the north of Germany and on the meadows of Holstein. Similar examples might be cited in regard to the Alpine-Arctic butterflies.
It is intelligible enough that we are still very far from being able to give a precise account of the main changes in the plant and animal world during the history of the earth in regard to the special causes which have produced them. Possibly the future will throw more light upon this subject by extending our knowledge of the fossil remains of all countries. But so much at least we can say at present, that there is no reason to refer the dying out of the earlier forms to anything else than the changes in the conditions of life, the struggle for existence, and the limitation of the power of transformation and adaptation due to the organization which the species has already attained; there is no trace of any such thing as a phyletic principle of life in the vitalistic sense, as far as the decadence of species is concerned.
LECTURE XXXVI
SPONTANEOUS GENERATION AND EVOLUTION:
CONCLUSION
Spontaneous generation—Experimental tests impossible—Only the lowest and smallest forms of life can be referred to spontaneous generation—Chemical postulates for spontaneous generation—Empedocles modernized—The locality of spontaneous generation—Progress of organization—Direct and indirect influences causing variation—The various modes of selection—Everything depends upon selection—Sinking from heights of organization already attained—Paths of evolution—The forces effecting it—Plasticity of living matter—Predetermination of the animate world—Many-sided adaptation of each group—Aquatic mammals and insects, parasites—Nägeli's variation in a definite direction—Analogy of the traveller—Genealogical trees—The diversity of forms of life is unlimited—The origin of the purposeful apart from purposive forces working towards an end—The limits of knowledge—Limitation of the human intelligence by selection—Human genius—Conclusion.
We have now reached the end of our studies, and they have given us satisfaction, at least in so far that they have brought us certainty in regard to the chief and fundamental question which can be asked in reference to the origin of the modern animate world of organisms. There remains no doubt in our minds that the theory of descent is justified; we know, just as surely as that the earth goes round the sun, that the living world upon our earth was not created all at once and in the state in which we know it, but that it has gradually evolved through what, to our human estimate, seem enormously long periods of time. This conclusion is now firmly established and will never again become doubtful. The assumption, too, that the more lowly organisms formed the beginnings of life, and that an ascent has taken place from the lowest to the higher and highest, has become to our minds a probability verging upon certainty. But there remains one point which we have not yet touched upon—the problem of the origin of these first organisms.
There are only two possibilities: either that they have been borne to our earth from outside, from somewhere else in the universe, or that they have originated upon our earth itself through what is called 'spontaneous generation'—generatio spontanea.
The idea that very lowly living organisms might have been concealed within the clefts and crevices of meteorites, and might thus have fallen upon our earth and so have formed the first germs of life, was first formulated by that chemical genius, Justus Liebig. It seems certain that the state of glowing heat in which meteorites are, when they come into our atmosphere, only affects the outer crust of these cosmic fragments, and that living germs, which might be concealed in the depths of their crevices and fissures, might therefore remain alive, but nevertheless it is undoubtedly impossible that any germ should reach us alive in this way, because it could neither endure the excessive cold nor the absolute desiccation to which it would be exposed in cosmic space, which contains absolutely no water. This could not be endured even for a few days, much less for immeasurable periods of time.