148 THE DOCTRINE OF DESCENT. 



ginning of this century, we may point even now to the 

 poverty of the collections. Wherever the palaeontologist 

 now lays his hand he finds intermediate forms, and day 

 by day the material accumulates as it is required. Never- 

 theless, too much is demanded, and the conditions of 

 preservation are misunderstood, if it be supposed that 

 all intermediate forms that ever existed were, by their 

 bodily constitution, either wholly or partially adapted to 

 preservation, and must therefore have been actually pre- 

 served. On the contrary, the greater number have as- 

 suredly vanished without a trace. At least half of all 

 geological deposits have been destroyed again during 

 slow upheavals. For from the time at which a sea- 

 bottom formerly lying at a profound depth, with its well- 

 preserved enclosures, is again raised within^ the reach 

 of superficial movements, it may be crumbled and cor- 

 roded, and the fossils contained in it now share the 

 fate which usually befalls the remains of the denizens of 

 flat shores, th^y are triturated by the surf. 



To this must be added the important consideration 

 that the forms by which the transition is effected will 

 mostly, not as individuals, but as forms, have had a briefer 

 period of existence than the persistent varieties appear- 

 ing to us as species, as may be seen, among other in- 

 stances, in the instructive discoveries at Steinheim. In 

 this particular, the periods of transition from one geo- 

 logical plane to the next, resemble the boundary regions 

 of two geographical districts. The tract of transition 

 from one to the other is specially suited to give rise to the 

 transformation of appropriate organisms. But this trans- 

 formation is accomplished and established first in the new 

 district. Thus in the geological series, transitional periods 



