EXTINCTION OF TYPES. 81 
of Echinoderms (Cystoids and Blastoids), many giant 
Reptiles, and some Mammals. 
It is almost certain that there has been no sudden 
extinction of any animal type. There is no evidence of 
universal cataclysm, though local floods, earthquakes, and 
volcanic eruptions occurred in the past, as they do still, 
with disastrous results to fauna and flora. In many cases 
the waning away of an order, or even of a class of animals, 
may be associated with the appearance of some formidable 
new competitors ; thus cuttle-fish would tend to exterminate 
Trilobites, just as man is rapidly and often inexcusably 
annihilating many kinds of beasts and birds. Apart from 
the struggle with competitors, it is conceivable that some 
stereotyped animals were unable to accommodate them- 
selves to changes in their surroundings, and also that some 
fell victims to their own constitutions, becoming too large, 
too sluggish, too calcareous,—in short, too extreme. 
Appearance of animats tn time.—Such tables as those given here are 
apt to be misleading, in that they convey the impression that the great 
types of structure have appeared suddenly. It must be noted that any 
apparent abruptness is merely due to incompleteness of knowledge or 
inaccuracy of expression. The table is a mere list of a few important 
historical events, but one miust fully realise that they are not isolated 
facts, that the present lay hidden in the past and has gradually grown 
out of it. Of the relative length of the periods represented here we 
know almost nothing, and we are also ignorant of the earliest ages in 
which life began. But the general result is clear. We find that in the 
Cambrian rocks, before Fishes appeared, the great Invertebrate classes 
were represented, though as yet but feebly. As we pass upwards they 
increase in number and in differentiation, Again, Fishes precede 
Amphibians, Amphibians are historically older than: Reptiles, and many 
types of Reptiles are much older than Birds. In short, in the course 
of the ages life has been slowly creeping upwards. 
[TaBLes. 
