170 MEANING OF TILE SYSTEM. 



residua which they contain have been completely destroyed, or so 

 altered that they cannot be recognised. 



In any case it may be regarded as certain, that only a small part of 

 the extinct animal and vegetable world has been preserved in a 

 fossil state, and that of this we only know a small part. Therefore 

 we cannot conclude that, because the fossil remains of intermediate 

 stages cannot be found, they have never existed. 



It is true that transitional forms are wanting in the strata where 

 they should have occurred, that a species suddenly appears in the 

 middle of a series of strata and suddenly disappears, and that whole 

 groups of species make their appearance and quickly vanish, but the 

 value of these facts as arguments against the theory of selection 

 is diminished by the circumstance that in certain cases series of 

 transitional forms between more or less remotely related organisms 

 have been found, and that many species have been developed in 

 course of time as links between other species and genera ; and again, 

 that species and groups of species not unfrequently increase very 

 gradually till they attain an unusually wide distribution, extend 

 into later formations, and then gradually di appear again. Such 

 positive facts have a higher value when we consider the incomplete- 

 ness of fossil remains. 



It will suffice here to refer to the Ammonites and Gasteropods, 

 such as Valvata multiformis, as examples supplied to us by Palaeon- 

 tology of transitional forms which can be arranged in a gradual 



series. 



Relation of Fossil Forms with Living Species. The close rela- 

 tionship of the plants and animals of the present time to the fossil 

 remains of recent formations is a fact of great importance. In 

 particular, we find in the diluvial period and in the different tertiary 

 formations the ancestral forms from which numerous living species 

 are directly descended : and further the characteristic features of the 

 fauna of any particular geographical province in the present epoch 

 are foreshadowed by the fauna of the epoch immediately preceding in 

 the same region ; a fact which is proved by the fossil remains we find 

 buried in the most recent strata. 



Many fossil Mammalia from the diluvial period and the most recent 

 (pliocene) tertiary formations of South America belong to types of the 

 order of Edentata which are now distributed in that part of the world. 

 Sloths and Armadillos of immense size (Meyatherium, Megalonyx, 

 Glyptodon, Toxodon, etc.) formerly inhabited the same continent, the 

 mammalian fauna of which in the present day is so specially charac- 



