16 SEA-SHORE LIFE 
trilobites, more than 1700 species of which existed in the Cambrian 
and Silurian rocks, survives to-day; and the sea lilies, which once 
grew in vast colonies along our shores, are now represented by only 
a few rare species 1p the deep ocean. 
The older naturalists did not realize the vastness of the destruc- 
tion which this battle for life has wrought, and thought that each 
species was a separate creation that had existed unchanged since 
the beginning. We now know that species appear to be distinct 
one from another because the intermediate forms have died out; 
and the longer a once closely related group of species has existed, 
the wider do their differences become until we may look in vain 
for the ‘linking forms” which once connected all of them. 
For example, we know that the vertebrates are recent, and are 
descended from the far more ancient invertebrates. Accordingly, 
we find that there are but few breaks in the chain of forms rang- 
ing from the lowest fishes to the mammals. Indeed, the embryos 
of the highest mammals display gill slits in their necks, which soon 
close over, but which were of use throughout life in their fish-like 
ancestors. | When we study the invertebrates, however, we find 
wide gaps so that no one has yet been able to determine the rela- 
tionships of some of the greatest groups. 
Tor example, we can not tell how the sponges, jellyfishes, echi- 
noderms or worms may be related one to another, but on the other 
hand we have reason for the belief that crustaceans and insects are 
descended from worm-like ancestors, and that mollusks may pos- 
sibly have had a somewhat similar origin. 
It is interesting to observe that we meet with the same condi- 
tions among plants. ‘The algz and fungi show wide gaps which 
prevent one from arranging them in any connected series, whereas 
the flowering plants, which are certainly more recent and have 
descended from non-flowering forms, can be arranged in a fairly 
regular chain, leading gradually from the lowest to the highest. 
We must bear in mind, however, that recent studies by Bate- 
son, de Vries and others tend to show that new species may appear 
suddenly as the offspring of old and well known forms; and that 
these new and peculiar animals or plants may, through inheritance, 
perpetuate their new peculiarities. There may then be no inter- 
mediate or “linking” forms between the old and the new species. 
