140 



GEOGRAPHICAL DISTRIBUTION 



to indicate the nature of the more important facts 

 affecting the solution of these problems. One of 

 the principal means of throwing light on this sub- 

 ject must obviously be to consider by what means 

 animals and plants are able to disperse themselves 

 across the barriers at present existing. 



It is scarcely necessary to draw attention to the 

 facilities for diffusion possessed by animals endowed 

 Avith great locomotive powers, and especially, among 

 land-animals, by those having the power of flight ; 

 and in connection with this means of dispersal the 

 most important thing to note is that some animals, 

 which in the adult state have only feeble powers of 

 locomotion, are better endowed in this respect in 

 an earlier stage of existence. Such, for example, 

 are univalve and bivalve marine molluscs, which 

 are all developed from free-swimming larvae. 



But, besides the normal means of locomotion, there 

 are many other modes of dispersal which it is highly 

 important, with reference to the present inquiiy, to 

 take into account. First, there is the power of 

 winds as a distributing agent. The carrying power 

 of winds is known to be sufficient to bear along in 

 the air line dust across seas many hundreds of miles 

 in width ; and, that being the case, we have in that 

 agency alone an adequate means of accounting for 

 the dispersion of all plants propagated by minute 

 spores. For that reason the distribution of most 

 cryptogamic plants hardly forms part of the problem 

 under consideration, and is generally left out of 

 account by those who have devoted themselves to 

 this investigation. What, part winds may have 

 played in carrying the seeds of phanerogamous 

 plants across arms of the sea is a more doubtful 

 point ; but there are observations which show that 

 even for such seeds, especially when provided with 

 some kind of feathery appendage, winds may occa- 

 sionally serve as a means of transport for very long 

 distances. Thus, Berthelot records that after a 

 violent hurricane he saw an annual belonging to the 

 Compositae (Erigeron ambiguus), widely distributed 

 throughout the- Mediterranean region, suddenly 

 appear at various spots on the Canary Islands, 

 where it was previously unknown, so that there 

 could be hardly any doubt that the seeds had 

 been blown across from Portugal or North Africa. 

 Nevertheless, De Candolle has shown that seeds 

 provided with a pappus are not on an average 

 more widely distributed than those members 

 of the Composites which are not so provided, 

 so that such a case as that just mentioned must 

 be looked upon as quite exceptional. But it is 

 exceptional means or transport that is most im- 

 portant to consider with reference to the problems 

 of distribution. 



But, in the case of animals also, winds are a more 

 important means of transport than one might at 

 first suppose. Birds and insects are often blown 

 immense distances out of their course ; and to this 

 cause, for instance, is due the arrival every year of 

 American birds on the Bermudas. Insects have 

 been caught on board of ships upwards of 300 

 miles from land. Further, there are well-authenti- 

 cated cases of even crabs, frogs, and fishes being 

 carried long distances by storms ; and in this way 

 it is possible to account for the transference of fish, 

 &c. from one river-system to another. Still more 

 frequently, in all probability, are the eggs of such 

 creatures transported by this means. 



Next, marine currents also form, beyond doubt, a 

 highly important means o dispersal both for plants 

 and animals, and that in various ways. First, seeds 

 may float on the surface of the ocean, and be 

 carried by currents for hundreds of miles, and 

 become stranded on a distant shore still in a con- 

 dition fit for germination. The well-known experi- 

 ments of Darwin to determine tlu vitality of seeds 

 in sea-water first enabled us to appreciate the 



importance of this factor in the distribution of 

 plants. In one experiment he found that, out of 

 87 kinds of seeds, 64 germinated after an immersion 

 of 28 days, and a few survived an immersion of 137 

 days ; and in another, that, out of 94 dried plants, 

 18 floated for above 28 days ; and, combining the 

 results of the two experiments, he concluded that 

 14 plants out of every 100 in the flora of a country 

 might be floated by currents moving at the average 

 rate of the several Atlantic currents a distance of 

 924 miles, and might, on being stranded, furnish 

 seeds capable of germinating. 



But further, marine currents often carry on their 

 surface various kinds of natural rafts, which may 

 be the means of transport both for plants and 

 animals. In the polar regions icebergs and ice- 

 floes may serve this purpose ; and elsewhere 

 trunks of trees, and even fragments torn from the 

 land. Such fragments, forming small islands with 

 erect trees upon them, have been seen at a distance 

 of 100 miles from the mouth of the Ganges and 

 other rivers. Wallace points out that ocean 

 waifs of one kind or another are almost the only 

 means we can imagine by which land-shells can 

 have acquired the wide distribution for which they 

 are remarkable. These molluscs perish very readily 

 in sea-water, but, on the other hand, are very 

 tenacious of life in other'circumstances ; and this 

 tenacity of life obviously favours their chance of 

 being carried in chinks of floating timber, or other- 

 wise, across the ocean. 



Again, locomotive animals are very frequently 

 the means of dispersing both plants and other 

 animals. Seeds may be attached to the fleece or 

 fur of mammals or the plumage of birds, or may 

 be enclosed in clumps of earth clinging to the feet 

 or some other part of bird or beast, even of insects. 

 To Darwin we are again indebted for an instance 

 showing how likely a means of transport this is. 

 He informs us that he received from Professor 

 Newton the leg of a red-legged partridge ( Caccabis 

 rufa ) with a ball of hard earth weighing 6 ounces 

 adhering to it. The earth had been kept for three 

 years ; but when broken, watered, and covered by 

 a bell-glass, as many as eighty-two plants sprang 

 from it. Hooked fruits, such as those of agrimony, 

 geum, &c., and fruits covered with a viscous sub- 

 stance, like those of some thistles, mistletoe, and 

 others, are the most likely to be transported in 

 this way. It seems probable that aquatic birds 

 and water-beetles have been the means of distrib- 

 uting aquatic plants and fresh-water molluscs, 

 which are remarkable for their wide diffusion ; and 

 the spawn of amphibians and fresh-water fishes 

 may be conveyed from one body of fresh water to 

 another by the same means. 



Again, seeds with hard shells are known in many 

 cases to be capable of passing through the digestive 

 organs of birds uninjured ; and consequently fruits 

 enclosing such seeds, or, like the strawberry, covered 

 with them, may be devoured by birds in one place, 

 and deposited by them in a state fit for germina- 

 tion at another, hundreds of miles distant. And 

 what is of still more importance, seeds which would 

 be destroyed if they passed through the digestive 

 organs of a bird are quite uninjured as long as 

 they remain in the crop, where they may be re- 

 tained for twelve or eighteen hours ; and thus birds 

 killed with food in their *rop may be the means of 

 scattering seed which has travelled 500 miles. It 

 is obvious that the migratory habits of certain 

 birds are of great importance with reference to both 

 the means of transport just mentioned. Some 

 seeds retain the power of germination even after 

 passing through the digestive organs of ruminants. 

 There is a well-established case of a tree belonging 

 to the order Leguminosse having been introduced 

 into the West Indies through cattle brought from 



