ch. iv.] THE LAW OF EVOLUTION. 345 



number of calcareous segments, originally separable, become 

 integrated into the compact boxes which envelope the organs 

 of the head and thorax. A similar concentration occurs in 

 the spider, the bee, and the butterfly. In contrast with this, 

 we may profitably observe what goes on in many annuloid 

 worms, where the multiplication of segments by differen- 

 tiation results in the fission of the animal into two distinct 

 individuals, because the integrating power of the organism is 

 slight. 1 Similarly in the development of the higher crasta- 



1 Here, without prejudice to the general argument, I may call attention to 

 the very ingenious hypothesis propounded by Mr. Spencer, to account for the 

 origin of the annulose or articulated sub-kingdom of animals. According to 

 this hypothesis, any annulose animal is in reality a compound organism, each 

 of its segments representing what was originally a distinct individual. In 

 other words, an annulose animal is a colony or community of animals which 

 have become integrated into an individual animal. Strong primd facie 

 evidence of such a linear joining of individuals primevally separate is furnished 

 by the structure of the lowest annelids. Between the successive segments 

 there is almost complete identity, both internal and external. Each segment 

 is physiologically an entire creature, possessing all the organs necessarj' for 

 individual completeness of life ; not only legs and branchiae of its own, but 

 also its own nerve-centres, its own reproductive organs, and frequently its 

 own pair of eyes. In many of the intestinal worms each segment has an 

 entire reproductive apparatus, and being hermaphrodite, constitutes a com- 

 plete animal. Moreover in the development of the embryo the segments 

 grow from one another by fission or gemmation, precisely as colonies of com- 

 pound animals grow. At the outset the embryo annelid is composed of only 

 one segment. The undifferentiated cells contained in this segment, instead 

 of being all employed in the formation of a heterogeneous and coherent 

 structure within the segment, as would be the case in an animal of higher 

 type, proceed very soon to form a second segment, which, instead of separat- 

 ing as a new individual, remains partially attached to the first. This prjcess 

 may go on until hundreds of segments have been formed. Not only, more- 

 over, does spontaneous fission occur in nearly all the orders of the annulose 

 sub-kingdom, but it is a familiar fact that artificial fission often results in the 

 formation of two or more independent animals. So self-sufficing are the 

 parts, that when the common earth-worm is cut in two, each half continues 

 its life as a perfect worm, — as is above observed, in the text. Veiy signifi- 

 cant, too, is the fact that in some genera, as in chsetogaster, where the perfect 

 individual consists of three segments, there is formed a fourth segment, 

 which breaks off from the rest and becomes a new animal. 



All these facts, together with many others of like implication, point to the 

 i »nclusion that the type of annulosa has arisen from the coalescence, in a 

 i near series, of little spheroidal animals primevally distinct from one 

 another. How are we to explain, or classify, such a coalescence ? May we 

 not most plausibly classify it as a case of arrested reproduction by spontaneous 

 fission i In other words, whereas the aboriginal annuloid had been in the 



