PHENOMENA OF CONVERGENCE. 23 



foreign bodies, in short, characteristics which would 

 seem to justify the conclusion of relationship, are 

 merely the result of mechanical contrivances, of 

 the effects of outward circumstances upon hetero- 

 geneous organisms. 



That outward circumstances have co-operated 

 helpfully and determinatively in the development 

 of symmetrical animal forms, cannot be doubted 

 by any except those who cling to their belief in the 

 type theory. It follows from this that, to a certain 

 extent, all organs appearing in pairs owe their 

 origin to convergence. We should further have to 

 weigh those circumstances where, in nearly related 

 organisms, the same organs vary on one side ; this, 

 for instance, occurs frequently in the pincers of the 

 Crustacea. It is only when starting from conver- 

 gencies of this kind which may be called homoeo- 

 genetic, and, according to general supposition, are, 

 as it were, self-evident that we can pass on to 

 those phenomena to which the idea of approxima- 

 tion is specially applied in our day, the heteroge- 

 netic cases of convergence. A shell-less tropical 

 snail, Onchidium, has eyes on the numerous wart- 

 shaped protuberances on its back. Semper l makes 



1 Semper, Ueber Seliorgane vom Typus der Wirbelthierangen 

 auf den Riicken von Schnecken. Wiesbaden, 1877. 



