36 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



(d) Migration. 



By migration must be understood the gradual movement of living forms 

 toward a more favorable environment or their extension within such an 

 environment. Large accumulations of fossils, especially those of one kind 

 or a limited number of kinds, are commonly due to the relatively sudden 

 action of some force catastrophic in nature and results. Frequently this 

 happens when under the stress of conditions such as a severe storm, unusual 

 conditions of heat or cold, aridity or drought, shifting currents or introduc- 

 tion of great quantities of sediment into a body of water, the more mobile 

 forms are suddenly forced into unfavorable surroundings and perish. Such 

 catastrophes are not migrations, nor are they to be interpreted as revealing 

 conditions other than accidental. Great accumulations of fossil material 

 other than of such forms as habitually grow in masses, beds, or reefs, as 

 corals or bryozoa, shell-beds, or forest accumulations, are to be looked 

 upon as unnatural and interpreted with much care. 



True migrations are movements in mass of a group of animals or plants 

 and are induced and checked by extrinsic factors. Even in the case of the 

 periodic movements of birds and grazing animals, or the more sporadic 

 movements of the lemmings, locusts, etc., the annual or periodical recur- 

 rence of the instinct of movement is revived by external factors. 



In animals the migration may be active or passive. Increasing numbers 

 may cause a peripheral pressure which will force individuals ever farther 

 from the original seat of the group until checked by impassable barriers of 

 some kind.^ Such a migration is generally in one direction and positive 

 in character, i. e., there is no advance and retreat, as in the "migration" of 

 birds, grazing animals, etc. It may be relatively sudden, as when a barrier 

 is removed from in front of a group experiencing strong peripheral pressure, 

 as when the Isthmus of Panama was formed or the Behring Straits closed, 

 or when a land barrier between two bodies of water is broken down; or 

 it may be slow and regular, as when a climatic change converts a plain 

 into a forest region or vice versa, or when a sea creeps over the land. 



Passive migration occurs where forms sedentary in the adult stage are 

 free in the egg, embryonic, or young stages and are then borne by water or 

 other currents to new regions. In this case the animals will go wherever the 

 current goes and will persist and leave traces wherever the conditions are 

 favorable. The same principle applies to forms free in the adult stage. 

 This type of migration is the common thing among the marine invertebrates 

 and is the form Ulrich has in mind when he announces his belief that little 

 evolution has taken place within the epicontinental seas, but mostly in the 

 ocean basins when the seas withdrew and the fauna migrated back upon the 

 land when the sea returned. 



* Scott, W. B., The Isthmus of Panama in Its Relation to the Animal Life of North and 

 South America, Science, vol. 43, p. 113, 1916. 

 Matthew, W. D., CHmate and Evolution, Annals N. Y. Acad. Sci., vol. xxiv, p. 177, 1915. 



