no 



EVOLUTION AND GENETICS 



ment of six symmetrical pairs of arches, of which some are later 

 resorbed to bring about the adult condition, we can conclude only 

 that the six are there because they were the original source of those 

 that persist. Nature is not in the habit of producing useless struc- 

 tures, and the appearance of unnecessary structures such as these, 

 which are so very similar to the adult arches of the fishes, points 

 strongly to the derivation of the higher forms from fish-like 

 ancestors. 



The Recapitulation Theory. The same interpretation can be 

 applied to any of the evidences of relationship previously brought 



out. Pharyngeal clefts, 

 Meckel's cartilage, ves- 

 tigial structures, all such 

 apparently useless 

 things, can be inter- 

 preted only as remnants 

 of an ancestral condition, 

 and since these things 

 resemble functional parts 

 of existing organisms, 

 those organisms may 

 logically be interpreted 

 as near that ancestral 

 condition. 



This resemblance of 



embryonic stages to a 



of adults of 



classes has 



to the rcca- 



succession 

 different 

 given rise 



Fig. 62. — Diagram showing the structure of a 

 primary ocelkis. c, cornea; c.hy., corneal 

 hypodermis; rel., retina; n, ocellar nerve; p, 

 accessory pigment cell; r, rhabdom. (From 

 Comstock's Introduction to Entomology, with 

 the permission of the Comstock Publishing 

 Company.) 



pitulation theory, the 

 belief that in its individual development an organism repeats 

 the steps of phylogenetic development which gave rise to its kind. 

 The repetition is undoubtedly modified in many cases according 

 to the conditions of existence of the various species, but there is 

 every reason to believe that it is in general true. In the transition 

 from a single cell to a small group, to the hollow spherical blas- 

 tula, the sac-like gastrula, and on through such details of develop- 

 ment as have been mentioned, it is highly probable that in em- 

 bryological development, or ontogeny, we have before us a partial 

 record of past changes. 



