STRUCTURE AND LIFE-HISTORY OF A SPONGE. 35 
sponge. What then is the meaning of these phases, why all this 
complicated process, instead of the simple impress of the parental 
image on a young germ? The explanation which has the merit of 
being at once the simplest and the most rational, is that the 
various stages in the development of the individual mark the 
various stages in the history of the species ; they present us, in 
the course of a few days, with a summary very much abridged 
of the successive steps by which the organism, as it at present 
exists, was evolved in the course of ages from some simpler form 
of life. Thus to confine ourselves to the history of the sponge 
which we have now made our own, we may assume that its earliest 
ancestor was a simple cell, closely resembling an ordinary amceba, 
this amceba, after leading a wandering life, feeding and growing 
big, became stationary, folded its arms, to speak symbolically, with- 
drew them into itself, and formed a spherical ovum; this either 
with or without fusing with another individual previously, split 
into two, as amoebas in such circumstances do at the present day, 
but the resulting twins, instead of separating from one another, 
as ordinary young amoebas do, remained in contact, for no obvious 
reason that one can see unless to keep each other warm; they 
grew up till the time came for them to split as their parent did, 
and thus four cells were produced, and so the process con- 
tinued, all the young cells sticking together on the principle of 
co-operation. But co-operation by itself will not produce very 
great results ; it frequently, however, leads to something much 
more important, and that is the specialisation of function and 
the differentiation of parts ; here, in the cluster of young amcebas, 
such a specialisation took place, some became set apart as food 
providers and agents in locomotion, their pseudopodia becoming 
converted into flagella (one wishes one knew how), the others 
served some other purpose, perhaps of secretion, perhaps as 
storers of nutriment, and perhaps as reproductive agents. So far 
the corals, sea-anemonies, and such like creatures (coelenterata) 
p 2 
