individual organisms, it goes on both longitudinally and transversely: under which different forms we may indeed most conveniently consider it. Of _longitudinal integration_, the sub-kingdom _Annulosa_ supplies abundant examples. Its lower members, such as worms and myriapods, are mostly characterized by the great number of segments composing them: reaching in some cases to several hundreds. But in the higher divisions—crustaceans, insects, and spiders—we find this number reduced down to twenty-two, thirteen, or even fewer; while, accompanying the reduction, there is a shortening or integration of the whole body, reaching its extreme in the crab and the spider. The significance of these contrasts, as bearing upon the general doctrine of Evolution, will be seen when it is pointed out that they are parallel to those which arise during the development of individual _Annulosa_. In the lobster, the head and thorax form one compact box, made by the union of a number of segments which in the embryo were separable. Similarly, the butterfly shows us segments so much more closely united than they were in the caterpillar, as to be, some of them, no longer distinguishable from each other. The _Vertebrata_ again, throughout their successively higher classes, furnish like instances of longitudinal union. In most fishes, and in reptiles that have no limbs, the only segments of the spinal column that coalesce, are those forming the skull. In most mammals and in birds, a variable number of vertebræ become fused together to form the sacrum; and in the higher quadrumana and man, the caudal vertebræ also lose their separate individualities in a single _os coccygis_. That which we may distinguish as _transverse integration_, is well illustrated among the _Annulosa_ in the development of the nervous system. Leaving out those most degraded forms which do not present distinct ganglia, it is to be observed that the lower annulose animals, in common with the larvæ of the higher, are severally characterized by a double chain of ganglia running from end to end of the body; while in the more perfectly formed annulose animals, this double chain becomes more or less completely united into a single chain. Mr. Newport has described the course of this concentration as exhibited in insects; and by Rathke it has been traced in crustaceans. During the early stages of the _Astacus fluviatilis_, or common cray-fish, there is a pair of separate ganglia to each ring. Of the fourteen pairs belonging to the head and thorax, the three pairs in advance of the mouth consolidate into one mass to form the brain, or cephalic ganglion. Meanwhile, out of the remainder, the first six pairs severally unite in the median line, while the rest remain more or less separate. Of these six double ganglia thus formed, the anterior four coalesce into one mass; the remaining two coalesce into another mass; and then these two masses coalesce into one.