THE DISTRIBUTION OF ANIMALS AND PLANTS.

Otago Witness, Issue 1892, 15 May 1890, Page 41

THE DISTRIBUTION OF ANIMALS AND PLANTS.

Naturalists now pretty generally admit that the area inhabited by a species is continuous, and if any very great barrier comes in between two areas inhabited by the same species the circumstance is considered peculiar. A wide expanse of ocean, for example, is an impassable barrier for a terrestrial animal, so that we seldom find the same species naturally inhabiting regions separated by wide ocean tracts. This is worthy of note, and lends great weight to the development theory, for it is scarcely conceivable that the animals found inhabiting two distant regions where the conditions of li:e are almost identical should have been specially created for these regions on widely dissimilar plans. Some of the plants found in regions wide apart are often identical, but this can be accounted for fiom the greater facilities possessed by plants for dispeision. It is not therefore a wild guess to assume that species originated in some one region, and were dispersed by natural processes far and wide over the earth as opportunity offered, undergoing modification during the process. The few isolated cases in which we find the same species inhabiting widely separated regions might appear to prove the contrary, but it is clear that we cannot command all the evidence bearing upon this vexed question. What changes of the land surface may have occurred in past time we cannot now even so much as guess. Where wide expanses of ocean sweep now continents may have at one time existed, and some think that the archipelagoes that in places stud the sea are but the mountain tops of submerged continents. Before we can therefore attempt to combat the idea that the animals and plants of the globe have sprung from some common centre, we would require to have an absolute knowledge of the distribution of land and water for all past time.

We have every reason to believe that many legions now completely separated were not always so, and that there were islands, now submerged, between continental areas that, seemed as convenient halting places for plants and animals iv their migrations. Exactly how these migrations took place we cannot, tell ; but by studying the various means by which migration is brought about in our own time we may arrive at a better understanding of the manner of the migrations of the past. That most careful observei-, Darwin, made a series of experiments to test how far seeds could resist the influence of sea water without having their vitality destroyed, and from a large number of observations conclusively proved that they could resist its destructive influence for periods of time sufficient, to allow of their being carried over considerable areas of ocean. He found, as the result of one experiment, that G4 kinds of seeds germinated out of 87 kinds immersed in sea water for 28 days, and a few survived an immersion of 137 days. It was found that some orders were far morp susceptible to injury than others. The Leguminosas were found to be easily injured, while some seeds of allied orders were all killed by a month's immersion 1 ; ' Conceiving that seed-bearing branches must often have been carried out to sea by floods, he next dried branches and tested their endurance in sea water. No less than 94 plants with ripe fruit were so treated. Some sank very soon, while others floated for 90 days, and when planted afterwards germinated. An asparagus plant with ripe berries floated for 23 days, and when dried it floated for 85 days, and the seeds afterwards germinated. Another plant, whose ripe seeds sank in two days, when dried floated for 90 days, and afterwards germinated. Altogether, out of 94 dried plants 18 floated for above 28 days, and some of the 18 floated for a very much longer time. From these experiments Darwin concluded that about one-seventh of the seeds of any country might be floated and carried by sea currents during 28 days. Now, as the average rate of the principal Atlantic currents is estimated at 33 miles a day, some being as swift as 60 miles a day, seeds might be floated across an expanse of ocean a thousand miles wide, and if carried by an inland gale to a favourable spot would afterwards germinate. Another observer who tried similar experiments found that about onefifth of the seeds of different kinds floated for 42 days, and were afterwards capable of germination.

There is yet another way in which seeds might be carried across wide ocean expanses, and that is by the aid of drift timber. Floating trees are occasionally cast up on even the most isolated islands. In some of the coral islands of the Pacific the natives procure tho only stones fit for weapons from among the roots of such drift trees, which are always welcomed with delight. Besides stones the roots of floating trees often hold portions of earth, and out of one such portion of earth, enclosed by the roots of an oak tree about 50 years old, Darwin found that three dicotyledonous plants germinated. Again, the seeds in the crop of a dead bird will retain their vitality for a long time. Peas and vetches are killed by a few days' immersion in sea water, but some taken out of the ciop of a pigeon that had floated on artificial sea water for 30 days nearly all germinated. And yet again living birds can hardly fail to be effective agents in the trunsportation of seeds. Birds are frequently blown by gales to distant regions far across the ocean. Some of the harder kinds of seeds pass unharmed through the digestive canal of birds. Darwin states that he picked up in his garden 12 kinds of seeds in the excrement of small birds, and some of them which were tiied germinated. The most important facl, however, bearing upon the distribution of seed by the agency of birds is that the crops of birds do not secrete gastric juice, and therefore do not in the least injure the germination of the seeds. If, as it is positively asserted, the seeds do not pass ~into the gizzard until after an interval of 12 to 18 hours, it is easily conceivable that in that, tiraea bird might be carried far across the sea in n, storm, and, landing on n hostile shore, fall a ready prey to hawks, when, ns

a matter of course, the do'ntents of their crops would be scattered. And yet once again some kinds of fresh water fish eat seeds of many kinds of land and water plants ; birds devouring these fish might carry the seeds from one place to another. Darwin says :— " I forced many kindd of seeds into the stomachs of dead fish, and then gave their bodies to fishing eagles, storks, and pelicans ; these birds after an interval of many hours either rejected the seeds in pellets or passed them in their excrement, and several of these seeds retained the power of germination." These are some of the best known moans by which the flora of one country might be transported to another widely separated by an ocean expanse. We need not be surprised theiefore at finding that tracts of land far apart may contain similar or almost identical plant life.