18 PRIMITIVE ANIMALS 



kingdom. Much as the geological record teaches 

 us and may yet teach us, it does not seem to touch 

 the origins of the animal phyla. The strata in which 

 those archives were deposited, if they ever were 

 deposited, have been crushed and burnt and re- 

 crystallized out of all recognition and their secret 

 is lost perhaps for ever, and we cannot even guess at 

 the nature of it. 



In what sense, then, can we speak of existing 

 animals as primitive or as affording links with the 

 past ? In the following chapters we propose to deal 

 with three classes or types of facts which throw a 

 light upon the past history of life. In the first place 

 there are animals still in existence which, although 

 clearly belonging to some recognized animal group, 

 have not advanced to the same stage of specialization 

 in all directions as their related forms, but have 

 retained in many respects a more generalized plan 

 of structure ; they have, so to speak, stood still on 

 the evolutionary road while the main troop of their 

 kind has travelled on and ultimately diverged into 

 many and various paths. In many cases we can trace 

 the geological history of such animals and we find 

 them existing at some very remote epoch practically 

 in the identical form which they exhibit to-day. Why 

 it is that certain animal types should remain so con- 

 stant for such long periods, while others related to 

 them should undergo the most fundamental changes, 



