410 PROCEEDINGS OF SECTION D. 
fixity of species. Natural selection, however, rests upon a suffi- 
ciently secure basis and is not really weakened because it does not 
afford an explanation of the mutual sterility of allied species. 
This infertility is, however, a most important fact in biology, and 
I think the explanation was found by Gulick* and Romanes.+ 
These authors advanced evidence to show that as variations 
occurred they became relatively infertile, first with the parent 
stock, and, secondly, with some of the variations among them- 
selves. 
The causation of this progressive infertility is not explained ; 
but Gulick’s observations leave little doubt as to its existence. 
If the operation of this factor be granted, then as variations 
became more markedly distinguished one from another, a corre- 
sponding degree of infertility might be expected, and this must 
inevitably lead to a polytypic evolution. 
The assurance that structural variation, as seen in all the different 
groups of animals and plants, is to be interpreted by the principle 
of community of descent, has once more attracted the attention of 
morphologists to the problems of growth and heredity, and an all- 
important question to them at the present time is the mechanism 
underlying these phenomena. ‘These are essentially physiological 
problems, and nothing illustrates the complete coincidence in point 
of view between modern morphology and physiology than the 
stand-point from which anatomists have attacked these questions. 
Morphologists seem to be so assured that these problems are to 
be explained in terms of mechanism as almost to make a simple 
physiologist who has hitherto worked at problems more obviously 
chemical or grossly mechanical, shiver at their temerity; for while 
the physiologist does not desire to cry a halt at this point, the 
known laws of chemistry and physics seem so hopelessly incapable 
of furnishing any interpretation of such things. 
Darwin, who fully realised this, put forward with diftidence his 
theory of pangenesis as a merely working hypothesis. The last 
ten years has seen much controversy anent heridity, and the now 
well-known theory has been advanced by Weismann to explain it. 
This theory recalls the old preformation theory of the seventeenth 
and eighteenth centuries, put forward by Haller and Bonnet, 
according to which the germ cell contained in miniature the 
whole structure of the adult which was gradually unfolded. 
Weismann observed that in Sagitta certain cells of the embryo 
are early set apart to form the germ cells of the next genera- 
tion. The germ plasm is thus in direct continuity through suc- 
cessive generations. 
According to Weismann and His even the earliest segmenta- 
tion cells of an embryo possess distinct and specific potentialities. 
* Jour. Linn. Soc. Zool. xix, 337. + Nineteenth Century, January, 1887, p. 61. 
