Xo. rm] 



EVOLUTIONARY THEORY 



455 



disappeared for ever. ... Not an exception is known to the rule. 



Even an organ once rudimentary, like flying in some 

 ground birds, never returns to full activity. A striking 

 example of dropping out is that of cilia in Arthropods; 

 which, ubiquitous in other groups of animals, are in this 

 group gone throughout; they fail to develop even in 

 spermatozoa. Toward the end of the process of evolu- 

 tion, a character tends to break up into a great number 

 of new characters. These usually affect a certain organ, 

 like the suture of the ammonites, and precede extinction 

 of the phylum. We may say that the number of genes 

 has become, through fractionation, so great that the re- 

 sulting complexity or the resulting extremes of develop- 

 ment of a trait are prejudicial to the development of the 

 organism. 



(d) Parallelism of Evolution in Allied Lmes.-This 

 parallelism was recognized by Darwin who writes : 



The principle formerly allu- 



oftei 



3 of the s 



•ited 



under similar exciting cause; 

 viously aid in the acquirem 

 organs, strikingly like each 



Of such parallel variations Osborn (1915, p. 216) speaks 

 as follows: 



Similar rectigradations may arise in all the descendants of similar 

 ancestors at different periods of time; they always give nse to P arallel - 



Illustrations of such are afforded in many paleontological 

 monographs. 



3. Another line of evidence for the theory of the 

 primacy of internal factors of evolution is found m ex- 

 perimental breeding. This evidence appears m the facts 

 (a) that manv mutations begin small and can be rapidly 

 evolved into highlv developed characters, (b) that simi- 

 lar variations appear in related organisms, (c) that 



