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- 
