148 THE DOCTRINE OF DESCENT. 
ginning of this century, we may point even now to the 
poverty of the collections. Wherever the palzontolo- 
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 atrace. 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 
