252 EVOLUTION AND GENETICS 



certain to result in interchange between divisions of the region 

 concerned which amount to a hmited migration. 



The opportunities available to a species within a limited region 

 depend partly upon its established habitat. A terrestrial form 

 is intennediate between fossorial and arboreal habitats, and 

 between aerial and aquatic. Any terrestrial form may avail itself 

 of the advantages of partial occupation of one or more of these 

 neighboring habitats without relinquishing its primary terrestrial 

 associations. A species living in the shallow sea is in a position 

 to enter the abysses or fresh water streams, or to move into the 

 intertidal zone of the littoral fauna. Such species are also subject 

 to transition between the three stages, benthos, nekton and plank- 

 ton. The more specialized forms, such as fossorial, abyssal and 

 benthonic animals, have more limited contacts and less ability 

 to respond to varied conditions. Specialization is fundamentally 

 a reduction of possibilities, since it demands particular fitness 

 of the organisms' equipment for some limited mode of life. In 

 spite of this limitation, however, even specialized forms retain 

 some latitude of response and diversity of contacts. It is only 

 in the most extreme forms, such as the endoparasites, that com- 

 plete inability to modify the mode of life in some slight degree is 

 likely to be found, and even some of these develop immunity to 

 drugs used for their destruction. 



Adaptive Radiation. The results of animal response to the 

 various possibilities of a limited environment have been expressed 

 in Osborn's law of adaptive radiation (Fig. 150). This law was 

 formulated in connection with studies of mammalian evolution, 

 but is generally applicable with minor modifications. As Lull 

 notes, "adaptive branching" is a more satisfactory term, since 

 the process is not always one of radiation from a common central 

 point. The principle involved is, in any case, much as it was 

 formulated by Osborn in his original law. It may be stated in 

 terminology as near as possible to the original as follows: Each 

 isolated region, if large and sufficiently varied in its topography, 

 soil, climate, and vegetation, will give rise to a diversified fauna 

 in any group of animals. The larger the region and the more 

 diverse the conditions, the greater the resulting variety of forms. 

 From a primitive stem form, new lines of adaptation will go out 

 into associated habitats. One result is divergence of form in 

 related animals, which has already been mentioned. 



