Likenesses; or, Philosophical Anatomy 259 



and between hand and foot, are hardly to be explained by l 

 any mere action of the environment. But serial homology 

 is much better exemplified in a very different group of 

 animals from backboned creatures — namely, in that group 

 to which all insects, lobsters, centipedes, leeches, and earth- 

 worms belong — the group of Annulose animals. In the 

 centipede, the body (except at its two ends) consists of a 

 longitudinal series of similar segments. Each segment 

 supports a pair of limbs, and the appendages of all the 

 segments (except at each end of the body) are completely 

 alike. In most other creatures of the Annulose group, the 

 fundamental similarity between the successive segments 

 of which the body is composed, is more or less disguised. 

 Thus, for example, in the lobster, a number of the anterior 

 segments of the body are united together into one solid mass, 

 while only in the abdomen (the so-called tail) do the seg- 

 ments remain distinct. The limbs also, which at first are 

 all similar, assume, with the development of the young 

 lobster, different forms, and become respectively antennae, 

 jaws, claws, legs, and swimming-feet. The peculiar and 

 strongly marked serial homology of these Annulose animals 

 has been the subject of an exceedingly ingenious suggestion 

 by Mr. Herbert Spencer. In his work just referred to, he 

 has attempted to explain such serial homologies thus: — 

 Some animals of a very low grade propagate themselves 

 by spontaneous fission — one individual spontaneously divid- 

 ing, and so becoming two distinct individuals. If certain 

 creatures found benefit from this process of division remain- 

 ing incomplete, they would (on the theory of ' Natural Selec- 

 tion') transmit to their posterity a naturally selected tendency 

 to such incomplete division. It is conceivable that certain 

 animals might thus have come to assume the form of a chain 

 of similar segments — i.e. a chain of imperfectly separated 

 individuals. Such a chain would, of course, in one kind of 



