BIOLOGY OF THE SCOTTISH LOCHS 
295 
their diurnal migrations to indicate great sensitiveness to minute 
variations of light, and the littoral species may have the same powers. 
DISTRIBUTION 
One of the departments of biology which it is hoped will be in 
some degree advanced by the labours of the Liake Survey is the study 
of distribution. Complex and difficult as is the investigation into the 
causes of the distribution of animals and plants over the globe, such 
as it is at the present day, vast accumulations of facts are needed 
before we are in a position even to tackle the problems with any 
hope of successful solution. 
The part played by climate in bringing about the present dis- 
tribution, the gradual changes of climate, the modification of species 
under this influence, the present and past forms of the continents and 
oceans, the alterations of sea-level, the facilities for transference 
between more or less widely separated regions, and in recent times 
the influence of man since commerce became universal, — all together 
render the problem of distribution one of the most complex and 
difficult, yet for that reason one of the most fascinating, studies for 
naturalists. 
If generalisations are attempted from insufficient data, very 
erroneous conclusions may be made. In no branch of natural 
history can it be said that the materials available are adequate for 
the exhaustive study of distribution. It thus happens that the 
commonest drudgery of the local naturalist, the mere collector's work 
and the tabulating of species, if conscientiously and accurately done, 
gains an importance and interest mayhap undreamt of by the col- 
lector. The abundant records obtained by the Lake Survey, though 
got haphazard while other work was being done, at all seasons of the 
year, and without any definite plan in which the biology was con- 
sidered, nevertheless present a serviceable mass of information. Each 
record, with its locality, is a definite fact, which can be used and 
assigned its proper place by special students of distribution. 
The more delicate problems of distribution will not be solved by 
mere compilers of lists of species in different regions of the world. 
Each naturalist gives his own value to the terms "species" and 
" variety," and if every man's contribution to the structure be com- 
piled without knowledge, the results may not justify the trouble. 
The valuation of the lists of species will have to be made, for each 
group, by men who know the whole group, and will know the values 
of species and variety, and the various shades of affinity within the 
group. While waiting for this ideal system of studying distribution, 
we must make our catalogues as best we may, and compare them 
