INTRODUCTION 



lations or on a whole community, and it in- 

 itiates and diiects the course of action of 

 innumerable small-scale events. Phenomena 

 on the largest scale may likewise depend 

 directly on the physical environment, as ex- 

 emplified by isostasy, the condition of equi- 

 Ubrium in which the heavier portions of the 

 earth's crust sink to form the ocean basins, 

 while the lighter parts are pushed up as the 

 continental platforms. 



The definition of ecology as the science 

 of communities may be vahd in its total 

 implications. The isopod illustration pre- 

 sents a phase of a much larger problem. In 

 another example, is the cell, the tissue, or 

 the organism as a whole the unit? The cell 

 may itself be broken into parts, and in 

 genetics we hear much about chromosomes, 

 chromomeres, and genes. So in ecology 

 there may be ecological relations of parts of 

 organisms— the nephiidial system, for exam- 

 ple—of the whole animal, of populations, 

 whether aggregated or dispersed, of asso- 

 ciations and communities, and of biomes. At 

 whatever level one begins, and whatever 

 the point of view, one must study all pos- 

 sible unitary levels before coming to a full 

 understanding of the ecology of either an 

 isolated isopod moving slowly upstream in 

 a small brook, or of the vast biome in which 

 the brook itself is a minor and almost neg- 

 Ugible incident. 



Close interaction exists between genes 

 and the general environment, both in devel- 

 opment and in evolution. A gene may be 

 helpfully regarded as a reagent in the proc- 

 ess of development; the environment also 

 enters intimately into the developmental 

 processes. Aside from supplying continuity 

 under suitable conditions, much that is pro- 

 duced by the gene system can be dupli- 

 cated by appropriate surroundings, either as 

 a result of shock furnished by an environ- 

 mental insult or from the more steady pres- 

 sure of a steadily continuing physical or 

 biotic induction. Such subjects are treated 

 in some detail in any modern work on phy- 

 siological genetics (Goldschmidt, 1938), in 

 more specialized books such as Hogben 

 (1933) or Newman, Freeman and Holzin- 

 ger (1937), and even in more popular ac- 

 counts, as in the small book by Dunn and 

 Dobzhansky (1946). 



Animals do not develop without an en- 

 vironment; contrariwise, even given opti- 

 mum environment, organisms do not start 

 to grow without the presence of a spore or 



zygote or of a group of cells from a pre- 

 ceding organism. Both a bearer of heredity 

 and a suitable environment are necessary for 

 development. After much discussion, lasting 

 from the time of Darwin, Galton, and 

 Weismann, we can now ask fairly exact 

 questions in this field and expect to find 

 fairly exact answers. Some pertinent data 

 are available at various evolutionary levels 

 such as those of the micro-organisms, the 

 insects, and man. The relation between 

 heredity and environment is frequently call- 

 ed the problem of nature versus nurture. 

 In its present dress the discussion does not 

 center about environment versus heredity in 

 general, but rather concerns the functions 

 of these two necessary components with 

 regard to some particular characteristic, 

 such as the color of the shanks in hens, the 

 width of the bar in bar-eyed Drosophila, 

 coat color in certain mammals, or intelli- 

 gence or stature in man. 



Concrete examples may clarify what is 

 meant by the ecological relations of such 

 characters. Yellow fat in rabbits or yellow 

 shanks in hens require a source of yellow 

 coloring matter, such as is furnished by yel- 

 low corn or by the xanthophyll from green 

 foliage or other similar foodstuffs; but, for 

 yellow to be developed, the enzyme that 

 breaks down xanthophyll must be absent, 

 and this lack in the hen or rabbit is asso- 

 ciated with gene action. Absence of xantho- 

 phyll from the food yields equally white fat 

 or white shanks, and one cannot know 

 whether the absence of yellow is primarily 

 environmental or genetic, or both, without 

 more direct knowledge of both the heredity 

 and the feeding routine. The effect of tem- 

 perature on the width of the bar in bar- 

 eyed Drosophila, of heat on the production 

 of feathers in young frizzle fowl, or of the 

 absence of iodine in water containing frog 

 tadpoles fed on an iodine-free diet, all dem- 

 onstrate significant effects of the environ- 

 ment on the development of characters 

 that are also definitely related to the gene 

 complex (Hogben, 1933). 



In man, the best assay of nature in asso- 

 ciation with or in contrast to nurture has 

 come from studies of identical twins reared 

 apart compared with those of others reared 

 together, and further compared with similar 

 qualities in fraternal twins. Identical twins 

 have an identical gene pattern, fraternal 

 twins do not. A good study of this kind is 

 that of Newman, Freeman, and Holzinger 



