In a preceding passage a clear recognition of the normal course of development occurs. In opposing the view set forth in [§§ 205–7] of this work, Prof. Korschelt says:—
“It seems scarcely favourable to this theory that the degree of independence which the individual segments present is comparatively slight. The most important organs (nervous system, body musculature, blood-vascular system) show themselves to be single fundaments of the entire body, and are also developed as such even though they also exhibit evidences of metamerism. Even the excretory canals may give up their segmental isolation and become united to one another by means of longitudinal canals.” (Ib. p. 348.)
On turning back to [§ 206], the reader will, I think, demur to the assertion that the independence is “comparatively slight”; seeing that, as in Ctenodrilus, a single segment sometimes becomes separate and reproduces other segments to form a new series. Instead of admitting that “the most important organs” “show themselves to be single fundaments of the entire body,” it may be held, contrariwise, that their original independence in each segment is masked only to the degree involved by their co-operation as parts of a compound organism. But chiefly I remark that when it is said that “the excretory canals may give up their segmental isolation and become united” by “longitudinal canals,” there is a clear confession that the isolation of these organs was original and their union superinduced—an implication that the course of evolution is as I have described it, and at variance with the course of evolution assumed by Prof. Korschelt.
Yet another incongruity is involved in his interpretation. He writes:—
“Just as in the consideration of the tapeworm chain we were induced by the comparison with unsegmented forms to refer the entire chain to an unsegmented individual, and, on the other hand, to see in the proglottis, not a complete individual, but only the abstricted hinder portion of the body of the Cestode, in the same manner, and with much more reason, we adhere to the individuality of the Annelid body.” (P. 349.)
And then on the preceding page, referring to the composition of the Annelid body, he says:—“The most natural comparisons are those with the tapeworm chain and with the strobila of the Scyphomedusæ.” Now since it is here assumed that the tapeworm and the strobila are analogous in composition, it is implied that the detached proglottis and the detached medusa are analogous; and hence if we are to regard the proglottis as “not a complete individual but only the abstricted hinder portion of the body of the Cestode,” then we must similarly regard the medusa as not a complete individual, but only the abstricted hinder portion of the strobila. This commits us to the strange conclusion that whereas individuality is ascribed to the original simple polyp, and by and by to the partially-segmented strobila, though these are without special senses and with only rudiments of muscular and nervous systems, individuality is denied to the detached medusa, which has organs of sense, a distinct nervo-muscular system and a considerable power of locomotion, as well as a generative system: traits which in other cases characterize developed individuals. Here also, then, there seems to be an inversion of the ordinary conception.
This conception of the proglottis and the medusa is, I see, accepted by some as tenable. But if we accept it we must accept also an analogous conception, which will I think be regarded as untenable. It is that supplied by the Aphides. From an egg proceeds a series of sexless and wingless females, and at the end of the series there come winged males and females with resulting gamic reproduction. If instead of forming a discrete series the imperfect females formed a concrete series, the members of which could individually feed without being detached from one another, as the segments of a tapeworm can, the parallelism would be complete; and then, according to the view in question, we should have to regard the perfect males and females eventually arising, not as individuals but as terminal portions of the series, containing generative products and having wings for the dispersion of them—locomotive egg-bearing segments of the chain. Whoever espouses this view must hold either that the first imperfect female of the series was the individual or that the entire string of them constituted the individual (in conformity with a view once propounded by Prof. Huxley). But he must do more than this. Since the Aphides have descended from some winged species of the order Hemiptera, he must hold that among those remote ancestors each particular fly, male or female, was an individual; but that when abundant food and inert life led to the partheno-genetic habit, and to chains of sexless forms, the males and females eventually produced at the end of each chain, though, like their remote ancestors, possessed of procreative organs and wings, are not individuals.
[Some memoranda bearing on the question here discussed, mislaid at the time when the chapter dealing with it was revised, have been discovered in time for utilization in this appendix.]
One of my critics says:—
“You have overstated the case in your favour: the alimentary canal does not, as you suggest, show a segmentation corresponding to that of the other organs in Annelids. Either it is a simple uniform tube, or else its differentiations (pharynx, œsophagus, crop, intestine) are quite independent of the repetition of the somites.”