LIFE HISTORY 35 



say nothing of the great mortality among those that find conditions 

 unsuitable for growth before maturity is reached. Only a few are 

 able to grow to maturity under this indirect method of selection. 



Among the multicellular motile species, with certain notable excep- 

 tions illustrated by a few fishes with pelagic eggs, the eggs or young 

 are very carefully placed in a suitable situation by the parent, and 

 the account of the life history properly speaking begins with the 

 breeding adult. However, land plants and animals differ fundamen- 

 tally as to nutrition, motion, factor control, and complexity, with the 

 result that in detail their respective life histories have little or noth- 

 ing in common. It thus becomes necessary to deal separately with 

 the two groups, though obviously this does not preclude taking into 

 account the intimate relations between plant and animal in the course 

 of their respective cycles. 



Relation to the Habitat. The relation of the life history to the 

 habitat is direct and intimate in the case of plants, a fact reflected 

 in the close correspondence of the two cycles during the year and 

 embodied in the phenomena of seasonal appearance, or phenology. 

 Such a connection is inherent in the process of aspection, in accord- 

 ance with which both plants and animals exhibit a seasonal rhythm 

 of appearance, growth, and reproduction. It is likewise concerned, 

 though to a less striking degree as a rule, in the related process of 

 annuation, in which presence and number are modified by the climatic 

 cycle. 



As indicated more fully later, hibernation and estivation are to 

 be regarded as peculiar types of aspection, while diurnation is a some- 

 what similar process operating in a daily cycle. Thus, the flowering of 

 a dandelion shows not only an annual and seasonal maximum, but 

 a marked daily period as well. The metamorphoses and activities of 

 insects and other invertebrates belong in the same general category. 

 In organisms with very short life cycles, such as diatoms, physical 

 factors may bring about two or more maxima of reproduction during 

 the year. One of the most striking examples of direct correlation of 

 habitat and control of life history is that cited by Russell and Yonge 

 for periwinkles (1928:51). The species nearest the low-water mark 

 hatches out in a very early stage as swimmers, the one near the mid- 

 dle of the intertidal belt appears in a later swimming stage, and that 

 near the high-water line produces young like the adult and therefore 

 able to crawl over the exposed rocks at once. 



