ANIMAL LIFE. 5°! 



and flow of water and exposing itself to such variations of conditions 

 as might lead to fresh-water existence — to life on the land and thence 

 to air, and, on the other hand, a dispersal on the surface to pelagic life. 

 Again from shore-life or the original near-shore location, there might 

 be a contingent working into deeper seas, and still further to occupa- 

 tion of abyssal depths. Transfers from surface to sea bottom, and the 

 reverse, seem probable, in fact certain, for some groups and movements 

 of terrestrial groups of animals into water must certainly have taken 

 place. The peopling of all habitable corners of the earth then has 

 been a process of continual pushing out from original centers, a con- 

 stant, if unconscious, effort of animal life as a whole to occupy all 

 available space, to crowd the energy of vital force to the end of every 

 open channel, to follow every thoroughfare and explore every by-path 

 that might lead to nook or corner in the universe that could give sup- 

 port in any fashion. But seldom has any form of life started alone 

 on its travels, and hence the crowding for place, the 'pussy wants a 

 corner' need, the eternal jostling to get and keep that corner and its 

 opportunities, the 'struggle for existence' that has been the dominant 

 principle of life from the dawn of its creation. Clearly those forms 

 most successful in adaptation to new conditions must be those that 

 win in the race and which soonest give rise to a higher and more com- 

 plex form of existence. Whether life began in a single organism the 

 parent form for all the mighty train that followed until now, or 

 whether numerous organisms started independently, we can hardly 

 doubt that all were equally simple, and similar courses of modification 

 must have affected all. Moreover, in every case, we are warranted in 

 assuming that for all higher types of animals there was a probably 

 common ancestral form, and distribution over the earth must have been 

 accomplished from an initial center by succeeding generations. Fur- 

 ther, that for each particular subordinate group, family, genus or spe- 

 cies that now has extended distribution, we must assume dispersal from 

 the original home of the ancestral form. 



First, then, we had only aquatic life, and this element may have 

 been densely peopled before an effort was made to move ashore or to 

 seek dry land. All geological evidence shows enormous development 

 of aquatic life in early times, but obviously such forms were most likely 

 to be preserved. Land life may have been forced by the drying up of 

 stretches of water as well as voluntary migration. But what an impor- 

 tant change that from aquatic to terrestrial life — from water-breathing 

 to air-breathing ! What possibilities of expansion, growth and occupa- 

 tion in the new, untrodden sphere, in the valleys and hills of earth and 

 in the invigorating supply of air ! Up this highway have come not a 

 few of the great groups of animals. Some of the simplest protozoans 

 even discovered the track, and worms of various kinds have crawled to 



