220 PLANT-LIFE 



leaving not a trace of their existence. As it is now, so 

 has it been throughout the ages. The great majority of 

 life-forms have utterly perished. Such fossils as are 

 found are the casts, impressions, or petrifactions of a 

 minority of bodies that happen to have been deposited 

 in conditions favourable to fossilization. A tree-trunk 

 that falls into a rapid stream and is carried into a lake 

 may rapidly become covered with a deposit of sand. 

 It is more likely to become a fossil than a trunk lying in 

 a moist wood, where it is rapidly reduced to humus by 

 bacteria, fungi, and other agencies. Besides, there must 

 always have been a great number of creatures with 

 bodies too soft and perishable for preservation in fossil 

 form. We hardly expect to find fossil Amcebce, but we 

 look for fossil records of molluscs, whose calcareous shells 

 are more or less durable, and we are not disappointed 

 in our search if we happen to " work " strata where 

 they occur. The fossil record is not complete, yet it is 

 remarkably rich. Indeed, when we think of the pas- 

 sage of time, the stress of the ages, and the nature of 

 the forms of which fossils have been found, we cannot 

 fail to marvel at its richness. 



In respect to the gamut of animal life, the witness of 

 the rocks and their contained fossils has at least enabled 

 us to make some broad generalizations. We can speak 

 with certainty of " Ages " when certain types of animals 

 were dominant — of the " Age of Amphibians," the " Age 

 of Reptiles," the " Age of Mammals " — and we can see 

 how reptiles gained the supremacy over amphibians, 

 and the later mammals in their turn came into the 

 ascendant, and drove the reptiles into hiding, many 

 of them into oblivion. And in many ways fossils 



