AGREEMENT OF COMMUNITIES 307 



Some investigators have questioned the importance of vegetation 

 to animals and we note here that the distributions of plant and animal 

 species are not always correlated. If one refers to species of plants 

 and species of animals then the vegetation very often is not correlated 

 with the distribution of the animals. If on the other hand one means 

 that the plants are controllers of physical conditions, then vegetation 

 can be said to be of very great importance. 



Before discussing the problem of agreement between plant and 

 animal communities, it is necessary to state what is meant by agreement. 

 According to present developments of the science of ecology plant and 

 animal communities may be said to be in full agreement when the growth 

 form of each stratum of the plant community is correlated with the conditions 

 selected by the animals of that stratum. Questions of agreement are pri- 

 marily questions for experimental solution. Two types of disagreement 

 are to be expected. We may illustrate the first by a bog or marsh 

 community. Considering plants rooted in the soil we note that water 

 is secured from the soil by the roots and is lost through the leaves and 

 twigs. Accordingly since bog soil is unfavorable, due to the presence 

 of toxins or to other causes, plants growing in it do not secure water 

 easily even when the quantity of soil water is great. Such plants have 

 xerophytic structures (which tend to check the loss of water) developed far 

 beyond the requirements of the atmospheric conditions surrounding their 

 vegetative parts. It is improbable that the animals inhabiting a bog- 

 vegetation field stratum would select atmospheric conditions such as 

 produce equally xerophytic structures under favorable soil conditions. 

 We may therefore expect disagreement. The smaller plants such as 

 fungi, algae, etc., are related to the strata of soil and atmosphere exactly 

 as the smaller animals and as much disagreement is to be expected between 

 such plants and the rooted vegetation as between the rooted vegetation 

 and animals. It must also be noted that the xerophytic structures of 

 the plants of unfavorable soils may have important influence upon ecto- 

 phytic plants and animals and in part counteract the effect of favorable 

 atmospheric conditions. 



The second type of disagreement is represented by cases in which 

 the vegetation lags behind. We have already noted that on the clay 

 bluff (pp. 209-17) conditions become favorable for inconspicuous plants 

 and forest animals as soon as the growth of the pioneer vegetation gives 

 shade to the soil. In other cases woody vegetation remains in situations 

 where the conditions have become unfavorable for it and the less con- 

 spicuous plants and some of the animals have disappeared. We may 



