256 SECRETS OF ANIMAL LIFE 
mental architecture and mode of development, and 
is always an indication of blood relationship, either 
near or remote. The only readily available dis- 
cussion of convergence is Professor Arthur Willey’s 
important essay Convergence in Evolution (1911), 
from which we have taken several illustrations. 
Queerest of queer fishes is the sea-horse, Hippo- 
campus, often seen in aquaria, which hangs itself 
from, or supports itself on, seaweeds by means of 
a prehensile tail, which moves dorsoventrally, not 
laterally, as in other fishes. It has a rapidly 
vibrating unpaired fin on its back, and the peculiarity 
of rolling its large eyes independently of one an- 
other. Now, it is curious that the far-removed 
chameleon, which is a quaint arboreal lizard, should 
show the same sort of prehensile tail as the sea- 
horse, only more so, and the same independent roll- 
ing of the eyes. But, as Professor Willey points 
out, the pipe-fishes, which are related to the sea- 
horse, but have not prehensile tails, also show inde- 
pendently rolling eyes. Therefore the association of 
separately moving eyes and prehensile tail is rather 
a coincidence. The significance of the mobile eyes 
is in relation to the sluggish habits and the relative 
inflexibility of the body in sea-horse and chameleon 
alike. 
Take another example. With the exception of 
the American opossums and _ selvas, all living 
marsupials (pouched mammals) are natives of 
Australasia, where, by a geological change, their 
ancestors were insulated, and thus left to evolve 
