784 EVENING DISCOURSES, 
much changed from their original condition that they sometimes suggest false 
conclusions. Although the geological record is so imperfect, it is therefore 
the most hopeful source of information concerning the origin and relationships 
of the various forms of life around us. 
The first important discovery which drew general attention to fossils in 
this connection was that of the long-tailed bird, Archwopteryx, in the Upper 
Jurassic Lithographic Stone of Bavaria. «at the present day there are no links 
between birds and the reptiles from which they appear to have been derived; but 
Archeopteryxz forms a distinct link, and it dates back to the period when we 
may suppose that birds were just coming into existence. 
The next remarkable step which attracted wide notice was Prof. O. C. 
Marsh’s discovery of the genealogy of the horse in the Tertiary rocks of North 
America. He and Prof, Huxley showed how the modern one-toed horse, 
adapted for rapid motion over hard ground and for feeding on dry vegetation, 
could be traced back gradually to small four-toed ancestors which lived in 
marshes on succulent food. Later discoveries showed that camels, cattle, pigs, 
elephants, and, in fact, most of our familiar larger animals, might be traced 
back to small marsh-dwellers which could only be regarded as common ancestors. 
These genealogies seemed fairly simple, but, as studies proceeded and dis- 
coveries multiplied, the subject proved to be much more complex than was 
at first suspected. The remains of rhinoceroses, for instance, of several suc- 
cessive geological ages, have been found both all over the Old World and in 
equal abundance over North America. They can be traced back to the usual 
common group of little marsh-dwelling ancestors; but, as shown by the re- 
searches of Osborn and Abel, there are obviously so many separate lines of 
descent, both in the Old World and in North America, that it seems almost 
impossible to unravel them. In the Old World the successive links in each of 
these lines gradually acquired the characteristic horns, and the rhinoceroses 
have continued to flourish exceedingly, at least in the warmer regions. In 
North America, on the other hand, for some curious undiscovered reason, 
they all died out when the growth of their horns was only just beginning. 
Among links more recently discovered, none are more interesting than those 
between the lung-breathing sea-animals and their land-ancestors. For many 
reasons, there can be no doubt that the whales and porpoises originated at the 
beginning of the Tertiary period from mammals which at that time lived on 
the land. Recent discoveries in the lower Tertiary rocks of Egypt actually 
show that the whales of that date had a skull and teeth almost identical with 
those of some of the contemporaneous land-dwelling flesh-eaters; and when the © 
rest of the skeleton is found it will almost certainly prove to be much less 
completely adapted for sea-life than that of the whales and porpoises of to-day. 
This passage of land animals to a life in the sea has occurred several 
times. In Triassic rocks we are now beginning to find the semi-aquatic ances- 
tors of the Ichthyosaurs and Plesiosaurs, which lived in all seas during the 
age of reptiles. We shall soon be equally successful with the ancestry of the 
sea-crocodiles, Mosasaurs and Champsosaurs, in successive later periods of the 
same great age. ; 
The repeated evolution of animals of the same general shape and habit 
from a successive series of distinct ancestors is a very remarkable and signifi- 
cant phenomenon. In fact, the more links in the chain of life we find among 
fossils, the more evident does it become that there are some very definite laws 
or principles underlying the changes we observe, and that they have remained 
the same through all geological time. There is good reason to believe that 
races of animals have a natural term of life just as individuals are limited, 
and that if circumstances allow them to follow their complete career they 
exhibit stages. which may be appropriately named youth, maturity, decline, 
and old age. The case of the chambered shells, known as ammonites and their 
allies, may be cited. We have reason to infer that they began in early Paleozoic 
times as straight shells; then they became curved and loosely coiled; next closely 
coiled ; next repeating these processes in reverse order, and finally many of them 
ending as straight shells. The same progress from youth to second childhood is 
seen when the Dipnoan mud-fishes are traced through geological time : the latest 
survivors of the race are eel-shaped, just as their earliest members are supposed 
to have been. The existing true eels are another example of fishes in their 
