these successive changes are so minute that in the course of our historical period—say three thousand years—this progressive variation has not advanced by a single step perceptible to our eyes, in respect to man or the animals and plants with which man is familiar, we shall admit that for a chain of change so vast, of which the smallest link is longer than our recorded history, the biologists are making no extravagant claim when they demand at least many hundred million years for the accomplishment of the stupendous process."
I will not stop to criticise the assumption that the jelly-fish is a remote ancestor of man; but, accepting all his data, will simply inquire how far Lord Salisbury's conclusion is warranted by them. As introductory to the criticism, I can not do better than quote another passage from the early essay named at the outset: merely remarking that the physiologist referred to as adverse in 1852, would not be thus referred to now. After remarking that those who know nothing of the science of life may naturally "think the hypothesis that all races of beings, man inclusive, may in process of time have been evolved from the simplest monad, a ludicrous one," the passage continues:
"But for the physiologist, who knows that every individual being is so evolved—who knows, further, that in their earliest condition the germs of all plants and animals whatever are so similar, "that there is no appreciable distinction amongst them, which would enable it to be determined whether a particular molecule is the germ of a Conferva or of an Oak, of a Zoophyte or of a Man";—for him to make a difficulty of the matter is inexcusable. Surely if a single cell may, when subjected to certain influences, become a man in the space of twenty years; there is nothing absurd in the hypothesis that under certain other influences, a cell may, in the course of millions of years, give origin to the human race."
Suppose we pursue the comparison indicated in the last sentence. Lord Salisbury invites us to reflect on "the prodigious change" required to transform his hypothetical jelly-fish into a man. He appears never to have reflected upon "the prodigious change" which in a few months transforms the human ovum into an infant. The contrast in structure may not be absolutely as great, since, in the course of the change from infancy to maturity, there is not only increase of size but some increase of structural development. In their essentials, however, the two organizations are alike: differences of proportion and finish chiefly distinguishing them. Let us, then, compare the embryological changes with the evolutionary changes, in their amounts and in the times taken by them. The nine months of human gestation, more exactly stated, is 280 days, that is 6,720 hours or 403,200 minutes. Thus, then, the total change from the nucleated cell constituting the human ovum to the developed structure of the infant just born, is
- Carpenter, Principles of Comparative Physiology, p. 474.