DISPERSAL OF PLANTS 321 



some means or another it is able, not only to spread itself over 

 its own area of distribution, but also., when occasion offers, to 

 extend that area by surmounting its barriers. 



The lower terrestrial plants, such as fungi, mosses and ferns, 

 are dispersed by means of spores, which, protected by special 

 envelopes, may be widely distributed by the wind. In the 

 higher plants the spores, as agents of dispersal, are replaced by 

 seeds, wbich, usually still within the fruit, maybe carried on the 

 wind, floated on rivers or ocean currents, carried about entangled 

 in the hair or feathers of animals, or actually eaten and passed 

 out uninjured in the freces. A great many seeds and fruits are 

 specially modified in structure to secure their distribution in one 

 or other of these ways, and the study of such adaptations consti- 

 tutes one of the most interesting chapters in botanical science. 

 We need only refer here to such fruits as the blackberry, whose 

 succulence tempts the birds to eat them and carry the seeds, 

 safely enclosed in their hard protective envelopes, to long 

 distances ; the various kinds of burs with their hooks for entangle- 

 ment in fur and feathers ; the winged fruits of the maple, elm 

 and ash, and the thistledown of the tbistles, adapted for floating 

 on the wind. 



The dispersal of plants is in all cases passive and dependent 

 on external agencies, though sometimes aided by some purely 

 mechanical device in the plant itself ; in the case of animals it 

 may take place either passively or actively, by the exertions of 

 the animals themselves. 



Beginning with the marine fauna, we find that the larger 

 forms whales, porpoises, dolphins and fishes owe their dis- 

 persal mainly to their own active powers of locomotion, while 

 the smaller animals, especially the invertebrates, are largely 

 dependent in this respect upon oceanic currents. 



Even animals which, like the sponges and corals, are firmly 

 fixed to the sea-bottom in the adult condition, have free-swim- 

 ming larval forms (Fig. 164) whose own limited powers of 

 locomotion may, under favourable circumstances, be enormously 

 supplemented by the action of currents. Such larval forms, 

 from the point of view of dispersal, play the part of the spores and 

 seeds of plants, and they occur not only in cases where the adult 

 is strictly sedentary in habit but also where its powers of loco- 

 motion are limited, as in many worms, snails, crabs (Fig. 128), 

 star-fishes, brittle stars (Fig. 127) and sea-urchins. The 



B. Y 



