1899] ZTE, SCOPE OF NATURAL SELECTION Ig! 
cell, again occupying an internal position in the group, enlarges at the 
expense of the surrounding cells, and when it has attained a certain 
size ruptures from the capsule which surrounded it, extrudes two 
nuclear portions of its substance (polar bodies), and if one of the 
smaller active cells comes into contact, and fuses with it, it will 
commence a series of cell divisions accompanied by increasing growth, 
and develop into an adult hydra similar to its parent. This sexual 
mode of reproduction very rapidly supplants all other forms; it is 
probable, therefore, that there is some immediate advantage resulting 
to the organisms which reproduce in this way rather than by budding. 
The most obvious difference in these two methods is that there is a 
great reduction of tissue material, much less being required for this 
mode of development than the other; it is therefore less expensive to 
the parent organism. Apart from this there is the additional factor 
that it would be the most suitable for development, if direct climatic 
accommodation does not take place, owing to its being the best means 
of obtaining the requisite amount of variability. This reduction must 
presumably be largely quantitative and not qualitative, since we find 
that under very dissimilar conditions a complex hydra can be formed, 
provided portions of both ectoderm and entoderm are preserved. 
Now, where this sexual mode of reproduction arises, we have to 
consider a new set of conditions; we find that each individual 
appears to go through a stage of development, maturity, and decay, 
and that during maturity the reproductive power of the whole 
organism is best developed. 
Perhaps one of the most striking facts associated with the higher 
forms of life is that these three periods of growth, maturity, and 
decay in the whole organism do not correspond in time to similar 
periods in the several different parts of the organism in question. 
This fact appears to be universal in its application; how is it to be 
explained? Now, as I have already noted, the most marked differ- 
ence between unicellular and multicellular reproduction consists in the 
fact that the latter develops chiefly by a quantitative evolution from 
a cell which is quantitatively undifferentiated, while the former 
reproduce by splitting off a portion of their structure, so that in most 
particulars, except size, the parent and the offspring are identical. 
Now one of the peculiarities of development and growth in one of the 
higher organisms is Just this quantitative development, and we must 
assume that the morphological element is present, for it is inconceiv- 
able that actual differentiation of structure could arise without some 
structural difference for its starting-point. We are bound therefore to 
assume two positions as essential to development: (1) Some basis for 
the differences that are found in individual development which must 
be of a structural and not a physiological nature, whether we call 
them gemmules, physiological or morphological units, biophors or 
stirp; (2) that development consists largely in a reduplication of 
