414 Life and Immortality. 
Thus it follows that the animals of any given formation, 
and the plants as well, where the records are preserved, are 
more like those of the next formation below and of the next 
formation above, than they are like any others. This fact of 
itself is an inexplicable one. But if we believe that the 
animals and plants of any given formation are, in part at 
any rate, the lineal descendants of those of the preceding, 
and the progenitors, also in part at least, of those of the suc- 
ceeding formation, then the fact is readily comprehensible. 
So frequently confronted is the paleontologist with the phe- 
nomenon of closely-related forms, especially of animals, 
succeeding one another in point of time, that he is compelled 
to believe that such forms have been developed from some 
common ancestral type by some process of evolution. Upon 
no other theory can we comprehend why the Post-Tertiary 
mammals of South America should consist of edentates, 
llamas, tapirs, peccaries, platyrhine monkeys and other forms 
now characterizing this continent, while those of Australia 
should be exclusively referable to the order of marsupials; 
and on no other view can we explain the common occur- 
rence of transitional forms of life, filling in the gaps between 
groups now widely distinct. But, on the other hand, there 
are facts which point clearly to the presence of some other 
law than that of evolution, and probably of a deeper and 
more far-reaching character. No theory of evolution can 
offer a satisfactory explanation for the constant introduction 
throughout geological time of new forms of life, which do 
not appear to be preceded by pre-existent allied types. The 
graptolites and trilobites have no known predecessors, and 
leave no known successors. Insects appear suddenly in the 
Devonian, and spiders and myriopods in the Carboniferous, 
but all under well-differentiated and highly -specialized 
forms. With equal apparent suddenness the Dibranchiate 
Cephalopods show themselves in the older Mesozoic de- 
posits, and no known type of the Palzozoic period can 
be pointed to as a possible ancestor. And so does the 
