119 
the species having arrived simultaneously on previously barren areas, 
and not having yet lived there long enough to have become fully 
adult. That the specimens are not dwarfed is shown by the absence 
in a striking degree of the age factor, black, in their coloration; 
larger specimens, or egg-bearing specimens of the same size, are 
much darker elsewhere; and in thé case of Himerometra crassi- 
pinna, which was found in abundance, I have personally examined 
a series of over twenty taken many years ago at Singapore, all 
of which are approximately of normal size, and all much larger 
than are any in this collection. All the Singapore species belong 
to the Zygometridæ, Himerometridæ, Comasteridæ, and Antedonidæ, 
families in which the eggs are large and presumably develop 
quickly, as in Antedon bifida. The embryos soon after being freed 
develop into larvæ which almost immediately settle down and grow 
up into pentacrinoids. The species of these families are gregarious 
and live as a rule in great masses, usually composed of several 
species; it is not improbable that the young in one of these great 
masses falling between the arms of the adults and developing in 
the shade of those arms, would, as soon as the pentacrinoid stage 
was passed and the animal commenced its nominally free, though 
practically sessile, existence, find that the large spreading arms of 
the parents shaded them, and effectually prevented any food from 
reaching them. Hence, in one of these masses, all the young would 
be continuously killed off by an unavoidable and effective process 
of race suicide, and the life of the mass as a whole would be 
limited by the life of the individuals in it, as they would not be 
succeeded by young. When the life cycle of the individuals in a 
mass Was run, the whole mass would die and disappear, leaving 
no trace of their former abundance. Their young, produced just 
before death, might be supposed to recolonize the locality and 
supplant the old as they died; but it has frequently been noticed 
that pentacrinoids seldom attach to dead or motionless objects; 
hence, there would be no recolonization of a place oceupied by 
One of these masses by its young, though some of these young 
