That all articulate animals are thus composed from end to end of homologous segments, is, however, an accepted doctrine among naturalists. It is a doctrine that rests on careful observation of three classes of facts--the correspondences of parts in the successive “somites” of an adult articulate animal; the still more marked correspondences of such parts as they exist in the embryonic or larval articulate animal; and the maintenance of such correspondences in some types, which are absent in types otherwise near akin to them. The nature of the conclusion which these evidences unite in supporting, will best be shown by the annexed copies from the lecture-diagrams of Prof. Huxley; exhibiting the typical structures of a Myriapod, an Insect, a Spider, and a Crustacean, with their relations to a common plan, as interpreted by him. [Illustration: Figs. 177–186.] Treating of these homologies, Prof. Huxley says “that a striking uniformity of composition is to be found in the heads of, at any rate, the more highly organized members of these four classes; and that, typically, the head of a Crustacean, an Arachnid, a Myriapod, or an Insect, is composed of six somites (or segments corresponding with those of the body) and their appendages, the latter being modified so as to serve the purpose of sensory and manducatory organs.”[27] Thus even in the higher _Arthropoda_, the much greater consolidation and much greater heterogeneity do not obliterate all evidence of the fact, that the organism is an aggregate of the third order. Comparisons show that it is divisible into a number of proximate units, each of which is akin in certain fundamental traits to its neighbours, and each of which is an aggregate of the second order, in so far as it is an organized combination of those aggregates of the first order which we call morphological units or cells. And that these segments or somites, which make up an annulose animal, were originally aggregates of the second order having independent individualities, is an hypothesis which gathers further support from the contrast between the higher and the lower Arthropods, as well as from the contrast between the Arthropods in general and the Annelids. For if that masking of the individualities of the segments which we find distinguishes the higher forms from the lower, has been going on from the beginning, as we may fairly assume; it is to be inferred that the individualities of the segments in the lower forms, were originally more marked than they now are. Reversing those processes of change by which the most developed _Annulosa_ have arisen from the least developed; and applying in thought this reversed process to the least developed, as they were described in the last Chapter; we are brought to the conception of attached segments that are all completely alike, and have their individualities in no appreciable degree subordinated to that of the chain they compose. From which there