Amongst the laws of life, the most conspicuous and undoubted is this—that each species reproduces itself, that like begets like. This law our author cannot of course gainsay; but he appends to it another overruling law, that from time to time, at long intervals, the like does not beget the like, but the different and superior form of organization. In other words, the old law changes from time to time. Of this novel description of law he borrows the following illustration of Mr Babbage:—
Unquestionably, what we ordinarily see of nature is calculated to impress a conviction that each species invariably produces its like. But I would here call attention to a remarkable illustration of natural law, which has been brought forward by Mr Babbage in his Ninth Bridgewater Treatise. The reader is requested to suppose himself seated before the calculating machine and observing it. It is moved by a weight, and there is a wheel which revolves through a small angle round its axis, at short intervals, presenting to the eye successively a series of numbers engraved on its divided circumference.
"Let the figures thus seen, be the series 1, 2, 3, 4, 5, &c. &c., of natural numbers, each of which exceeds its immediate antecedent by unity.
"Now, reader," says Mr Babbage, "let me ask you how long you will have counted before you are firmly convinced that the engine has been so adjusted that it will continue, whilst its motion is maintained, to produce the same series of natural numbers. Some minds are so constituted, that after passing the first hundred terms they will be satisfied that they are acquainted with the law. After seeing five hundred terms, few will doubt; and after the fifty thousandth term, the propensity to believe that the succeeding term will be fifty thousand and one, will be almost irresistible. That term will be fifty thousand and one, and the same regular succession will continue; the five millionth, and the fifty millionth term will still appear in their expected order, and one unbroken chain of natural numbers will pass before your eyes from one up to one hundred million.
"True to the vast induction which has been made the next succeeding term will be one hundred million and one; but the next number presented by the rim of the wheel, instead of being, one hundred million and two, is one hundred million ten thousand and two. The law changes."
The illustration is carried through a page or two more, but we have quoted all that is essential.
Mr Babbage makes a very useless parade here of his calculating machine. A common household clock that strikes the hours, would illustrate all that his machine can possibly illustrate. If the reader seat himself before that homely piece of mechanism, he will hear it tick for sixty minutes, when the law of the machine will change, and it will strike.
In a scientific point of view it is absurd to talk about the law of his machine. His machine partakes only of the laws of mechanics, which, we presume, are as constant there as elsewhere. Our only definition of law is, a sequence that is constant; deny its constancy, and you deny it to be law; it is a mere contradiction in terms to speak of a law that changes.
If, therefore, our author, guided by this illustration of Mr Babbage's, proclaims a law of animal life which changes of itself from time to time, he is departing from the fundamental principle of all science—he who is so zealous to reduce all phenomenon to the formula of science! Anxious to escape from an abrupt interposition of creative power, he introduces a sudden mutability in the laws themselves of nature! If it be said that he does not (although his words imply it) insist upon a single law of nature that varies at intervals, but contends for a variable result, produced by the law of reproduction acting under varied circumstances, and in co-operation with different laws—then was Mr Babbage's machine of no use whatever to him, nor did he stand in need of any peculiar illustration. There is not a class of phenomena which does not exhibit this variety of result by the diversified co-operation of laws constant in themselves. The frozen river becomes motionless; it ceases to flow; yet no one attributes any inconstancy to the laws of heat, or the laws of hydrostatics.
Quitting these abstractions, in which the writer before has shown himself no very great adept, let us enquire by what arguments he attempts to support his peculiar principles of development. That on which he appears chiefly to rely is the fact, that the embryo of one of the higher animals passes through the fœtal stages of the lower animals—the fish, the reptile, the bird—before it assumes its last definite shape. From this he would infer, that the germ of life is alike in all, and that it depends only on peculiarities of gestation whether it shall become a fish, a fowl, or a mammal. He lays particular stress on the circumstance, that the brain of the human embryo passes through these several stages.