148 THE DOCTRINE OF DESCENT. 



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

 poverty of the collections. Wherever the palaeontolo- 

 gist now lays his hand he finds intermediate forms, and 

 day by day the material accumulates as it is required. 

 Nevertheless, 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 preserved. On the contrary, the greater number 

 have assuredly 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 

 corroded, and the fossils contained in it now share the 

 fate which usually befalls the remains of the denizens 

 of marshy shores, they 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 appearing to us 

 as species, as may be seen, among other instances, in 

 the instructive discoveries at Steinheim. In this parti- 

 cular, the periods of transition from one geological plane 

 to the next, resemble the boundary regions of two geo- 

 graphical districts. The tract of transition from one to 

 the other is specially suited to give rise to the transfor- 

 mation of appropriate organisms. But this transforma- 

 tion is accomplished and established first in the new 

 district. Thus in the geological series, transitional periods 



