INTRODUCTION. 7 



An instance of this kind in the animal kingdom 

 is the sea otter, connecting the seal with the weasel 

 family, and representing by a living animal, the 

 starting point of these two lines of evolution. In 

 the vegetable kingdom, one of the most striking 

 instances of a living link i3 the Cycas family, the 

 most ancient type of the Gymnosperms. This cu- 

 rious type connects the line of Gymnosperms with 

 that of the vascular Cryptogams, and even partakes 

 in a certain measure of some of the peculiarities of 

 the Monocotyledons, in characters which are per- 

 haps more than mere analogy, so that it really 

 forms a connection between fern, fir and palm. 



In most instances the connecting link is extinct ; 

 in this case two possibilities present themselves. 

 First, the type may be preserved in a fossil state, 

 as the Hippotherium connecting the horse and 

 camel, and among plants the singular fossil Lepi- 

 dodendron, a paradoxical type which combines the 

 characters of Lycopodium with those of the Coni- 

 fers — classes which to-day are widely separated. 



Second, the extinct type may have been entirely 

 lost, not even existing in a fossil state, so that it 

 can only be theoretically reconstructed by combin- 

 ing the characters common to both lines of evolu- 

 tion, and separating them from those peculiar to 

 each. Such an instance is the connecting link be- 

 tween birds and reptiles, types now separated by a 

 wide interval which is approached but not bridged 

 over by types found in a fossil state. There still 

 yawns a chasm between the most reptile-like bird 



