THE LAW OF EVOLUTIOX. 323 



the lower creatures to the higher. As manifested in either 

 way, 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 char- 

 acterized 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, acompanying 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 on 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 annulose animals. In the lobster, the head and 

 thorax form one compact box, made by the union of a num- 

 ber of segments which in the embryo were separable. Simi- 

 larly, 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 one another. The 

 Vertebrata again, throughout their successively higher 

 classes, furnish like instances of longitudinal union. In 

 most fishes, and in reptiles that have no limbs, none of the 

 vertebra* coalesce. In most mammals and in birds, a varia- 

 ble number of vertebrae become fused together to form the 

 sacrum ; and in the higher apes and in man, the caudal verte- 

 bra? also lose their separate individualities in a single os 

 coccygis. That which we may distinguish as trans- 



verse 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 com- 

 mon with the larva of the higher, are severally character- 

 ized by a double chain of ganglia running from end to end 



