ECOLOGICAL SUCCESSION. 339 



or even quaternary significance, even when details are being con- 

 sidered. This applies to particular mores (ecological species) 

 as well as to groupings of higher order. Ecological specificities 

 are primarily differences in physiological life histories manifested 

 mainly by (a) details of time and place of reproduction and degree 

 of latency in reproductive structures, and (b) by quantitative dif- 

 ferences in reactions to the same intensity of the same environic 

 factors. Because of lack of knowledge of life histories, the latter 

 will doubtless be most useful in practice. It is also the best 

 test of animals temporarily invading a community to which 

 they do not primarily belong. Such animals should be in 

 partial agreement with the communities which they have 

 entered even though their residence there be temporary. 



3. Stratification or Vertical Aspects. 



(a} Adaptation. In the preceding paper we divided the animal 

 communities into strata. Persons not familiar with ecology appear 

 to think that structural adaptations are an important part of the 

 consideration of modern ecology. In the first place, ecologists are 

 skeptical of the significance of many if not of the majority of so 

 called structural adaptations. In general, structural adaptations 

 appear not to be correlated with the phenomena with which the 

 modern ecologists are concerned. For example we note (Shel- 

 ford, '07, 'I2 1 ) four species of tiger beetles arranged in the hori- 

 zontal series of conditions which we find at the south end of 

 Lake Michigan. A careful study of the adults and the larvae of 

 these species fails to show any structural characters which are 

 correlated with the conditions in which the species live. All 

 have the same type of mandibles, the same kind of feet, and the 

 same kind of ovipositor. There are no structural characters 

 by which they can be located in their environments. The adults 

 are structurally adapted to making holes in the ground with 

 their ovipositors and thus depositing their eggs. The larvae are 

 adapted to a life in the ground. These are structural adapta- 

 tions to stratum. All terrestrial tiger beetle species are somewhat 

 in agreement as to adaptations. Other adaptations among the 

 tiger beetles are adaptations for walking on leaves of plants 

 (Odontochila, Bates, p. 169), for creeping on the trunks of trees 



