From all of which results the general conclusion, worked out more fully in my paper on the "Homologues of Organic Creatures," that here is the true route of organic development; the vertebrates from the mollusks, and these from the articulates proceed; and that the basis of the history we are seeking must be searched for in the history of segmentation.
All are familiar with the construction of an annulose ring or segment. To the rest of the compound creature each segment is a microcosm, precisly repeating similar parts, or their homologues. To each segment belongs a neural ganglion, or rather a pair of ganglia; a dorsal vessel swelling in the middle to a pulsating organ, or heart, or rudiments thereof; and each is furnished with stigmata, spiracles, or appendages, which are homologous of each other, or, as before, with rudiments thereof. And all these by sufficient analysis are to be traced as actually present in every true segment, however often the function of the homologous structure may be changed, or however it may be reduced to a rudiment, or atrophied, or absorbed and apparently lost. These homologues constitute the basis of biology, as they do of the life-functions of all creatures made of segments, which includes, as we have seen, mollusks and vertebrates, as well as annulosa—possibly, also, molluscoidea; unless the latter should be considered a modified cœlenterate, and then it would be the beginning and unit of the series. If, as sometimes contended (and these are unsettled questions), the molluscoid is an evolution of the annulose type, and its embryonic history tends to prove this, then all the segments have been lost but one. For the molluscoids are monosegmental. Nor is this proposition strange or improbable. Undoubtedly such a reduction to one segment takes place in high orders of genuine mollusks, as in gasteropods (of which you have instances in common snails), in whom one segment has become atrophied, leaving generally a rudiment behind.
In vertebrata, which more immediately concern us, two segments, and only two, are always present, and always bear with them their distinctive elements or appendages, however rudimentary some of the latter may sometimes appear. These segments really constitute the well-known bilateral arrangement of parts and organs so general in animals of this class. Here, again, it is necessary to go back a little in order that we may make the greater speed forward. It is necessary distinctly to understand what we are talking about, and what we mean by segments. In the discussion of the bisegmental organization of vertebrates, the question comes up whether any true homologues exist in the two great classes, vertebrata and annulosa. In the present advanced condition of biology, this question receives a decided affirmative. Until recently the contrary was generally supposed.
When it is seen that two sides of a vertebrate, if it be split asunder down the backbone, present exact counterparts of each other,
