PACTS AND OBSERVATIONS. xli 



migration. This is very liable to happen if two or three species (or genera) reach their maximum 

 of quantity and power at the same time and at the same place ; the equilibrium between want and 

 supply is destroyed. Deaths may exceed the births in number, from an unusually hot or cold 

 season, from scanty food, or from a change in the mineral ingredients in the sea ; it may have 

 become brackish, fresh, or too salt. Instances of such conditions abound. The operations of man 

 have but slight effect on marine life. 



MIGRATION. Any considerable removal of a living creature from place to place is called 

 migration, whether directly by its own act or not. It is called transport when life has ceased. 

 At present we speak only of the inhabitants of the Silurian sea. Migration has always been a 

 great fact, and must often occur to an animal of fixed wants living among varying conditions. It 

 colonizes unoccupied spots by swarming from crowded places, throws foreign life into old com- 

 munities, thereby conferring variety, perhaps together with some advantages. 



The processes of nature are in ceaseless operation ; portions of the sea- grounds are continually 

 being made unfit for the occupation of organic beings, and then again are restored to their use : 

 so it has always been. Here migration comes into play and builds up comparatively permanent 

 societies. A living individual is set in motion by external agencies ; and sooner or later it fixes 

 upon some one spot, there it remains, and it either dies, languishes, or prospers. Living, it spreads 

 by reproduction; meanwhile it is joined by other individuals in growing numbers. Some of these, 

 finding appropriate conditions, flourish, and a community is eventually established, which divides 

 and subdivides, and flits about (within narrow limits, it is true) in search of food, shelter, and other 

 necessaries. Communities (genera and species) take action, and remove when necessary ; but the 

 impulse is from the individual : it is in his interest. Migration in Silurian times must have been 

 hazardous, but more so now, when exterminating agencies are more numerous. 



In those times (as now) there must have been failure upon failure in changing their abodes, 

 down the measureless flow of time from the Silurian to the Permian formation; and we know of 

 generic forms which have made this long voyage. 



We usually trace the march of the migrant but imperfectly, but at other times pretty well, 

 from land to land, because it often forms settlements as it goes. 



Every free animal is by nature a wanderer in search of pasture and security, the lowest forms 

 having the greatest migratory power. This process may be in abeyance ; for a community may be 

 stopped by great depths, hedged in by high sea-cliffs or by sea-deserts of sand and shingle, 

 impassable, especially by the herbivor just as some of the Bohemian Trilobites occupy small 

 patches in the midst of an untenanted waste. Extent of dispersion is in proportion to these and 

 other obstacles, as well as to viability. Or the creatures may have been content with their quarters ; 

 for the individual stops on the instant that his wants are supplied, the next move perhaps being 

 made by his uneasy progeny. Communities sometimes leave their abodes all in a body : they are 

 either swept away by a high tide, or some such strong current, or, as must often happen, the 

 herbivorous division move away in search of food and shelter, and the carnivorous must go with 

 them necessarily. Each act of migration has its own direction, distance, and method, about which 

 something will be said in the sequel. 



Whole communities have been known to return together to the country they had long 

 abandoned. Mr. Godwin- Austen gives a remarkable instance of this kind of repossession in the 

 Palaeozoic rocks near Boulogne (France) . Here alternations of level have introduced into the same 

 area, successively, distinct assemblages of suitable marine life, one or two of them actually accom- 

 plishing a repetition of occupancy (Journ. Geol. Soc. Lond. Ix. 244). 



The only entirely satisfactory proof of a fossil having begun to exist in another place or 

 horizon than that in which it is first seen is its being so found ; but the following marks taken 

 together (more or fewer) will leave little room for mistake. The migrant is apt to be solitary, 

 with no kindred, young or old, around it. It may be in a coarse foreign sediment, travel- 



