LIFE AT THE SURFACE OF THE SEA. 197 
In trying to read the ancestral history of a species, two 
lines of evidence are open to us. Firstly, there is the 
embryological or developmental evidence. In the history 
of the development of the individual animal we frequently 
find strange phases of growth which have no reference to 
the present adult form, and which are only explicable on 
the recapitulation theory, the theory, that is to say, that the 
development of the individual is a brief epitome of the 
development of the race. Even in the adult there may be 
found structures and organs whose existence has no refer- 
ence to present needs, but points back to some ancestral form. 
Secondly, there is the geological evidence, the evidence 
preserved in the rocks of far-back time, in the fossil remains 
of the ancestors of living forms. 
Both these forms of evidence will assist us in obtaining 
some answers to our present question. Bearing these facts 
in mind, let us look again at the pelagic fauna and ask our 
question, “ Whence came these?” At once we are struck 
by the fact that many of the pelagic animals, admirably 
adapted though they are for a marine life, must be modified 
descendants of shore or land animals. The Polar bears, the 
seals, and walruses, even the more extremely modified 
whales, and all the sea birds are obviously simply land ani- 
mals which have become secondarily adapted to a marine 
life. Yet a very strange fact is that when we attempt to 
go further back still, we find in the embryological history of 
birds and mammals unmistakable evidence that these, in 
turn, were descended from some ancestors which were not 
mammals or birds at all, but which lived an aquatic life and 
breathed by means of gills. 
Among the invertebrata, again, we find some, such as the 
marine insect Halobates, which have obviously taken to a 
pelagic life at a comparatively recent period. 
Something of the ancestral history of fishes can be made 
out from the testimony of the rocks, The higher fishes of 
our own day, the bony fishes, appear first in the Secondary 
Period ; their ancestors are to be sought in a much earlier 
period; and as far back as the Silurian age, before the 
famous Old Red Sandstone was laid down, we find that 
Ganoids and Elasmobranchs were abundant. It is surely a 
