CONVERGENCE IN EVOLUTION 255 



This illustrates what is called convergence, the 

 occurrence of similar adaptations to similar con- 

 ditions in two sets of animals not even distantly 

 related. Fishes and cuttlefishes are on entirely 

 different lines of evolution; moreover, the in- 

 dividual development of the eye is radically different 

 in the two cases; yet both may show telescope 

 eyes. Weismann defined convergence as " corre- 

 sponding adaptations to similar conditions in ani- 

 mal forms not genealogically connected with one 

 another"; and, in addition to the unrelatedness of 

 fish and cephalopod, he pointed out that the fishes 

 with telescope eyes could not be regarded as the 

 descendants of a single ancestral species which 

 achieved the remarkable adaptation. It seems 

 rather that, even within the class of fishes, telescope 

 eyes have arisen independently several times over. 

 Similarly it may be noticed that the adaptation of 

 pectoral fins as volplanes must have occurred inde- 

 pendently in two distinct sets of fishes, and that the 

 transformation of muscular tissue into an electric 

 organ must have occurred independently, at least 

 twice, namely, in the Torpedo type and the Gym- 

 notus type, while that of the African catfish is on a 

 different line, being transformed glandular tissue. 

 Very much the same as " convergence " is the term 

 " homoplasy," which Sir Ray Lankester used for 

 similarity of form in types of quite different pedi- 

 gree. It is important for clear thinking to dis- 

 tinguish convergent or homoplastic resemblance 

 from homology, which means sameness in funda- 



