—_—_— —— -s 
SECTIONAL TRANSACTIONS.—D. 391 
Lastly, many naturalists had emphasised the difficulty of seeing how one 
structure presumably derived from another could have been evolved, except 
by chance mutations. He would remind them where, as in lineage series, 
the actual course of evolution was known, the functional nature of the slow 
gradual changes was evident. Let what we know not be shaken by what 
we do not know. 
Dr. R. GuRNEY. 
The distribution of fresh-water Entomostraca shows, in many cases, 
precise association of species with particular environments which can only 
be accounted for by almost unlimited means of dispersal and selective 
destruction. The trivial structural differences between pairs of species 
cannot have survival value, but must be accompanied by physiological 
differences which cannot be estimated. Evidence can be given of structural 
changes directly caused by change of environment, but these changes are 
so small that the direct influence of environment cannot be one of the main 
factors in evolution. 
Prof. H. L. Hawkins. 
Palzontology shows the history of the relation between organisms and 
environment. Most fossil evidence that is reasonably consecutive is found 
in marine organisms; and even there no genetic relationships can be 
proved. 
Given a constant environment, the tendency of the struggle for existence 
is to prevent the origin of species ; but the histories of the Micrasters and 
Gryphzas show that progressive change proceeds in defiance of this tendency. 
Such evolutional changes affect characters that are only incidentally con- 
nected with physiological efficiency, although they may prove fatal when 
carried too far. Such cases imply the existence of a directional momentum 
of change that is wholly intrinsic. 
Changing environment may cause migration or extinction ; but it cannot 
be proved to produce direct adaptive change. The correlation between 
organisms and their environments is most easily explained by the influence 
of habitat on structure; but in many cases this leaves the ‘ choice’ of 
environment as a problem. Any effect caused by environment is limited 
by the potentials of the organism, and is subordinate to the course of intrinsic 
change. 
Dr. W. K. Spencer, F.R.S. 
The species clusters are those arranged around evolving lines of starfish 
found in the chalk of Western Europe. Here there was fairly uniform 
physical conditions existing over a wide area for a very considerable time. 
Evolution was gradual and in definite directions. The changes involved 
are increase in diameter, in depth of body, and in modifications of shape 
and of ornament of marginalia. ‘These modifications are parallel in several 
independent lines. ‘The ornament characters could have had little func- 
tional importance. They are definitely related to the age of the lines and 
may be indices of physiological changes in the race. Branches from lines, 
giving new genera and species, seem to owe their new character in part 
because of acceleration in their racial history giving earlier maturity and 
premature senility. 
There is no evidence of suppression of lines by competition. Lines 
which have become rare and then disappear are those which have had a 
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