588 STRUCTURE AND LIFE HISTORIES 
number, both of specimens and of species, preserved in 
our museums," says Darwin, "is absolutely as nothing 
compared with the number of generations which must 
have passed away during a single formation." The 
meagerness of the record is, of course, due in part to the 
relatively small area explored in proportion to the whole; 
but there are other reasons much more serious, because 
they represent opportunities lost forever. Among them 
are metamorphosis, explained above, and the fact that 
many of the organisms of the past were composed wholly 
or partly of soft tissues, which were entirely destroyed, by 
decay or otherwise, in the process of rock-formation. 
Such plants, for example, as Spirogyra and many other 
algae, the fleshy fungi, and, among animals, jelly-fish, 
earthworms, and others, would form fossils only under 
exceptionally favorable circumstances, if at all. 
But there is an even more effective cause of oblitera- 
tion of the fossil record in the long-continued erosion and 
denudation represented by unconformity in the rock 
strata. In many cases only a small proportion now re- 
mains of the thickness of a rock stratum originally de- 
posited, and all traces of the plant and animal life that 
may have existed on the denuded area have thus been ob- 
literated forever. These blank intervals between suc- 
cessive periods were of vast duration. 
"I look at the geological record," said Darwin, "as a 
history of the world imperfectly kept, and written in a 
changing dialect; of this history we possess the last 
volume alone, relating only to two or three countries. 
Of this volume, only here and there a short chapter has 
been preserved; and of each page, only here and there a 
few lines. Each word of the slowly changing language, 
