Page:Popular Science Monthly Volume 43.djvu/491

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have quoted above, a crack in the doctrine is admitted: it is said that "this differentiation was not at first absolute, and indeed it is not always so to-day." And then, on turning to page 7-4, we find that the crack has become a chasm. Of the reproductive cells it is stated that—"In Vertebrata they do not become distinct from the other cells of the body until the embryo is completely formed." That is to say, in this large and most important division of the animal kingdom, the implied universal law does not hold. Much more than this is confessed. Lower down the page we read—"There may be in fact cases in which such separation does not take place until after the animal is completely formed, and others, as I believe that I have shown, in which it first arises one or more generations later, viz., in the buds produced by the parent."

So that in other great divisions of the animal kingdom the alleged law is broken; as among the Cœlenterata by the Hydrozoa, as among the Mollusca by the Ascidians, and as among the Annuloida by the Trematode worms.

Even in ordinary life, a man whose supposition proves to be flatly contradicted by observation, is expected to hesitate; though, unhappily, he very often does not. But in the world of science, one who finds his hypothesis at variance with large parts of the evidence, forthwith abandons it. Not so Prof. Weismann. If he does not say with the speculative Frenchman, "tant pis pour les faits," he practically says something equivalent:—Propound your hypothesis; compare it with the facts; and if the facts do not agree with it, then assume potential fulfillment where you see no actual fulfillment. For this is what he does. Following his admission above quoted, concerning the Vertebrata, come certain sentences which I partially italicize:

"Thus, as their development shows, a marked antithesis exists between the substance of the undying reproductive cells and that of the perishable body-cells. We can not explain this fact except by the supposition that each reproductive cell potentially contains two kinds of substance, which at a variable time after the commencement of embryonic development, separate from one another, and finally produce two sharply contrasted groups of cells" (p. 74).

And a little lower down the page we meet with the lines:

"It is therefore quite conceivable that the reproductive cells might separate from the somatic cells much later than in the examples mentioned above, without changing the hereditary tendencies of which they are the bearers."

That is to say, it is "quite conceivable" that after sexless Cercariæ have gone on multiplying by internal gemmation for generations, the "two kinds of substance" have, notwithstanding innumerable cell-divisions, preserved their respective natures, and finally separate in such ways as to produce reproductive cells. Here Prof. Weismann does not, as in a case before noted, assume