THE DISTRIBUTION OF ANIMAL LIFE. 95 



the simplest uaicellular animals still existing side by 

 side with the highest forms of life ; in the geological 

 record we may expect therefore to find evidence of 

 a similar state of things ; nor have we any reason to 

 suppose that the earliest fossiliferous rocks we are 

 acquainted with are the product of a period of time 

 in which only very simple forms existed. It is pro- 

 bable that the rocks which contained the record of 

 the earliest developments of life have long been 

 converted into what are called metamorphic rocks — ^ 

 rocks which, while they preserve traces of their 

 stratified origin, have been so altered in their intimate 

 structure by heat, pressure, and chemical action, that 

 the organic remains they formerly contained have all 

 been destroyed. Stratified rocks are constantly being 

 altered in this way ; and the older they are, the less 

 is the chance that they will have remained unaltered. 

 Moreover, it must be remembered that only animals 

 that possessed some hard structure are preserved in 

 a fossil state. The foraminifer, with its shell, lasts 

 almost for ever; but of cells like the amoeba, no 

 trace could remain embedded in a rock. It is there- 

 fore vain to hope to trace the earliest history of life 

 from the record of the rocks, although we find in 

 them the landmarks of its subsequent development. 



But since the simpler types survive still, the in- 

 teresting inquiry presents itself whether we may not 

 be able to discover life in the very making, find the 

 simplest type of all, and see whether its cells are 

 really always produced from a pre-existing germ of 



