CAUSES OF SUCCESSION 247 
Second stage (Fig. 248): Bark loosened; wood still solid or barely 
softened. Under the bark were the flattened Pyrochroidae larvae, the 
small snail (Zonitoides arboreus), a few of the four-legged larvae of 
the passalid (Passalus cornutus), many larvae of fungus-gnats (Myceto- 
philidae), and a single specimen each of the beetle (Penthe pimelia) and 
the slug (Philomycus carolinensis). None of these were abundant. 
The flattened beetle larvae were most characteristic. 
Third stage (Fig. 249): The wood is thoroughly softened and the 
bark generally loosened. Here the animals present in the earlier stage 
are increased in numbers. The passalid larva is more abundant. 
Slugs are numerous. Snails (Pyramidula alternata) are found in such 
situations as are large enough for them to enter. Fungus-eating beetles 
are present (Megalodacne heros Say). A click-beetle larva (Tharops 
ruficornis Say) bores into the softened wood. 
Fourth stage (Fig. 250): The bark fallen off; the log a mere mass of 
rotten wood. Such a log is only shelter for the regular inhabitants of 
the forest floor which we have already enumerated on the preceding 
pages. 
VI. GENERAL DISCUSSION 
A study of the tables shows several points of interest. Take first 
the ground stratum. Beetles which live under decaying wood are 
common on the beach where the decaying wood is common, but are 
absent through the cottonwood, pine, and black-oak stages. They 
appear again with the fallen leaves and moist logs of the black oak-red 
oak stage. Vegetation in itself is not directly important. Moist 
decaying wood is common, both on the beach and in the woods. Wood 
and moisture are evidently essential to such animals. Turning to the 
snails, which probably all come out into the open to feed during the night 
and during moist weather, we note that they do not appear until the 
under-log beetles put in their second appearance. In general the total 
number of species and of individuals increases until the oak-hickory 
stage is reached and falls off again in the beech and maple stage. 
In general we note that as the forest passes from the bare-sand stage 
to the beech-maple stage, there is a great increase in the space to be 
inhabited by animals and the diversity of possible habitats, at least up 
to the oak-hickory stage. 
I. CAUSES OF SUCCESSION 
The causes of succession in forests are chiefly changes in physical 
condition with increase in denseness of vegetation, such as the increase 
