184 THE GENESIS OF SPECIES. [CHAP. 



tendency to the development at intervals of a complex 

 whole. It is not the budding out or spontaneous fission 

 of certain segments, but the transformation in a definite 

 and very peculiar manner of parts which already exist into 

 other and more complex parts. Again, the processes of 

 development presented by some of these creatures do not 

 by any means point to an origin, through the linear coales- 

 cence of primitively distinct animals by means of imperfect 

 segmentation. Thus in certain Diptera (two-winged flies) 

 the legs, wings, eyes, etc., are derived from masses of form- 

 ative tissue (termed imaginal disks), which by their mutual 

 approximation together build up parts of the head and 

 body, 10 recalling to mind the development of Echiiioderms, 



Again, Nicholas Wagner found in certain other Diptera, 

 the Hessian flies, that the larva gives rise to secondary lar- 

 vae within it, which develop and burst the body of the pri- 

 mary larva. The secondary larvae give rise, similarly, to 

 another set within them, and these again to another " set. 



Again, the fact, that in Tcenia echinococcits one egg 

 produces numerous individuals, tends to invalidate the ar- 

 gument that the increase of segments during development 

 is a relic of specific genesis. 



Mr. H. Spencer seems to deny serial homology to the 

 mollusca, but it is difficult to see why the shell segments 

 of chiton are not such homologues because the segmenta- 

 tion is superficial. Similarly the external processes of eolis, 

 doris, etc., are good examples of serial homology, as also 

 are plainly the successive chambers of the orthoceratidae. 

 Nor are parts of a series less serial, because arranged spi- 

 rally, as in most gasteropods. Mr. Spencer observes of the 

 molluscous as of the vertebrate animal, " You cannot cut it 

 into transverse slices, each of which contains a digestive or- 

 gan, a respiratory organ, a reproductive organ, etc." 12 But 



10 Prof. Huxley's Hunterian Lecture, March 16, 1868. 



11 Ibid., March 18. 12 "Principles of Biology," vol. ii., p. 105 



