CHAP. VIII.] LIKENESSES IN ANIMALS AND PLANTS. 259 



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 segments remain distinct. The limbs 

 also, which at first are all similar, assume, with the develop 

 ment of the young lobster, different forms and become re 

 spectively jaws, claw?, 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 re 

 ferred to he has attempted to explain such serial homologies 

 thus : Some animals of a very low grade propagate them 

 selves by spontaneous fission one individual spontaneously 

 dividing, and so becoming two distinct individuals. If 

 certain creatures found benefit from this process of division 

 remaining incomplete, they would (on the theory of Natural 

 Selection) 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 animal be the equivalent of a series of perfectly 

 separated individuals of another kind of animal in which the 

 process of fission was completely carried through. In other 

 words, Mr. Spencer would explain the serial homology of 

 Annulose animals by the supposed coalescence (through im 

 perfect fission) of organisms of very simple structure, such as 

 the small aquatic worms called Planarias, in one aggregated, 

 longitudinal series through the survival of the fittest aggre 

 gation. This is a very ingenious speculation, yet not only is 

 there no evidence that Planarise propagate by fission, but 

 there is positive evidence which directly conflicts with Mr. 

 Spencer s hypothesis. Mr. Mosely, in his investigations of 



s 2 



