MIGRATION 59 



fishes, aquatic and gallinaceous birds, and some mammals afford 

 readily observable examples. Permanent aggregation by reproduction 

 occurs in ants and a few other social insects. Doubtless, sessile ani- 

 mals afford examples of aggregation by asexual reproduction, but aside 

 from the corals, noteworthy i)ermanent examples are not outstanding, 

 either in their conspicuousness or ecological significance. As Alice and 

 others have pointed out, the process of aggregation in motile animals 

 is dependent upon sexual forces, upon social forces, and upon common 

 environmental responses. Aggregation by reproduction as cited above 

 is essentially social. In animals with minimal social tendencies the 

 offspring disperse early or may be loosely held together by common 

 environmental responses (cf. Chapter 5). 



MIGRATION 



In spite of general agreement in the sense of movement, this term 

 has come to have somewhat different applications in botany and in 

 zoology. This has probably come about as a consequence of the 

 basic contrast in motility, so that it became desirable to distinguish 

 the distant or recurrent movement of animals in mass from local 

 activities. In sessile plants, any movement was of some significance, 

 but the most noticeable ones were those of the individual for a short 

 distance. Moreover, in terrestrial animals the adults are much more 

 motile than the young, while with plants the embryo or seed is often 

 very mobile and the adult immobile, except for the slow and re- 

 stricted spread of offshoots. 



The general dependence of the flowering plant upon the seed is 

 indicated by the word dissemination, which might well replace migra- 

 tion were it not for two facts. The first is that plants bring about 

 effective change of position by means of propagules (Fig. 13), and 

 the second that vegetation often exhibits great mass movements, in 

 which the associated animals are also involved. In consequence, it 

 seems most logical and convenient to employ migration to denote any 

 and all changes of position, whether of individual or community, single 

 or recurrent, over a restricted or local area, or for great distances. 

 The term would still retain its special application to seasonal, annual, 

 or cyclic movements in mass, characteristic of certain insects, most 

 birds, and some mammals and fishes. Dissemination would continue 

 to apply to the local transport of seeds and fruits in particular, 

 whereas migration would be especially applicable to mass movement 

 in response to climatic change, in which seeds, propagules, and motile 

 animals would all be involved (Clements, 1922:351). 



