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been produced by the direct action of the environment, 
and therefore must have been produced by Natural Selec 
tion ; or at least to indicate how it might have been 
accomplished by Natural Selection. 
In reference to the development of the amoeba, Mr. 
Spencer says :— 
“ The limiting membrane becomes in higher infusoria definite and 
often complex, showing that the selection of favourable variations has 
had largely to do with its formation.”—(/>. jj.) 
But why should the believer in Mixed Darwinism affirm 
that definite and complex structures must have been 
produced by Natural Selection, and take that for granted 
without attempting to discuss the direct action of the 
environment ? 
We have next to consider the process of invagination, 
by which the gastrula stage of development is attained. 
Mr. Spencer gives the following illustration of the 
process :— 
“ Take a small indiarubber ball, not of the inflated kind, nor of the 
solid kind, but of the kind about an inch or so in diameter, with 
a small hole, through which, under pressure, the air escapes. Sup 
pose that, instead of consisting of indiarubber, its wall consists 
of small cells, made polyhedral in form by mutual pressure, and 
united together. This will represent the blastoderm. Now with the 
finger thrust in one side of the ball until it touches the other, so 
making a cup. This action will stand for the process of invagination.” 
—(f. 64.) 
“ If we go back to the first stage in which Protozoa, having by 
repeated fissions formed a cluster, then arranged themselves into 
a hollow sphere, as do the protophytes forming a volvox (Mr. 
Spencer argues that), originally alike all over its surface, the hollow 
sphere of ciliated units thus formed would, if not quite spherical, 
assume a constant attitude when moving through the water; and 
hence one part of the spheroid would more frequently than the rest 
come in contact with nutritive matters to be taken in. A division of 
labour resulting from such a variation being advantageous, and 
tending, therefore, to increase in descendants, would end in a differ 
entiation like that shown in the gemmules of various low types of