4o6 



NATURE 



{Sept. 7, 1876 



but where we find a series of species of distinct genera, all differ- 

 ing from their continental allies in a whiter colouration, as in the 

 Andaman Islands and the West Indies ; and among butterflies, 

 in the smaller Moluccas, the Andamans, and Madagascar, we 

 cannot avoid the conclusion that in these insular localities some 

 general cause is at work. 



There are other cases, however, in which local influences seem 

 to favour the production or preservation of intense crimson or a 

 very dark colouration. Thus in the Moluccas and New Guinea 

 alone we have bright red parrots belonging to two distinct 

 families,' and which, therefore, most proliably have been inde- 

 pendently produced or preserved by some common cause. Here 

 too and in Australia we have black parrots and pigeons -^ and it 

 is a most curious and suggestive fact that in another insular sub- 

 region — that of Madagascar and the Mascarene Islands — these 

 same colours reappear in the same two groups." 



Some very curious physiological facts bearing upon the pre- 

 sence or absence of white colours in the higher animals have 

 lately been adduced by Dr. Ogle.* It has been found that a 

 coloured or dark pigment in the olfactory region of the nostrils 

 is essential to perfect smell, and this pigment is rarely deficient 

 except when the whole animal is pure white. In these cases 

 the creature is almost without smell or taste. This, Dr. Ogle 

 believes, explains the curious case of the pigs in Virginia 

 adduced by Mr. Darwin, white pigs being poisoned by a poison- 

 ous root which does not affect black pigs. Mr. Darwin imputed 

 this to a constitutional difference accompanying the dark colour, 

 which rendered what was poisonous to the white-coloured 

 animals quite innocuous to the black. Dr. Og'e however 

 observes, that there is no proof that the black pigs eat the roat, 

 and he believes the more probable explanation to be that it is 

 distasteful to them, while the white pigs, being deficient in smell 

 %nd taste, eat it and are killed. Analogous facts occur in 

 several distinct families. White sheep are killed in the Tarentino 

 by eating Hypericum criscum, while black sheep escape ; white 

 rhinoceroses are said to perish from eating Euphorbia candel- 

 abrum ; and white hories are said to suffer !rom poisonous food 

 where coloured ones escape. Now it is very improbable that a 

 constitutional immunity from poisoning by so many distinct 

 plants should in the case of such widely different animals be 

 always correlated with the same difference of colour ; but the 

 facts are readily understood if the senses of smell and taste are 

 dependent on the presence of a pigment which is deficient in 

 wholly white animals. The explanation has, however, been 

 carried a step further, by expeiiments showing that the absorption 

 of odours by dead matter, such as clothing, is greatly affected 

 by colour, black being the most powerful absorbent, then blue, 

 red, yellow, and lastly white. We have here a physical cause 

 for the sense-inferiority of totally white animals which may 

 account for their rarity in nature. For few, if any, wild animals 

 are wholly white. The head, the face, or at least the muzzle or 

 the nose, are generally black. The ears and eyes are also often 

 black ; and there is reason to believe that dark pigment is 

 essential to good hearing, as it certainly is to perfect vision. 

 We can therefore understand why white cats with blue eyes are 

 so often deaf — a peculiarity we notice more readily than their 

 deficiency of smell or taste. 



If then the prevalence of white colouration is generally 

 accompanied with some deficiency in the acuteness of the most 

 important senses, this colour becomes doubly dangerous, for it 

 not only renders its possessor more conspicuous to its enemies, 

 but at the same time makes it less ready in detecting the presence 

 of danger. Hence, perhaps, the reason why white appears 

 more frequently in islands where competition*is less severe and 

 enemies less numerous and varied. Hence, also, a reason why 

 albiiioism, although freely occurring in captivity never maintains 

 itself in a wild state, while melanism does. The peculiarity of 

 some islands in having all their inhabitants of dusky colours — 

 as the Galapagos — may also perhaps be explained on the same 

 principles, for poisonous fruits or seeds may there abound which 

 weed out all white or light-coloured varieties, owing to their 

 deficiency of smell and taste. We can hardly believe, however, 

 that this would apply to white-coloured butterflies, and this may 

 be a reason vhy the effect of an insular habitat is more 

 marked in these insects than in birds or mammals. But 

 though inapplicable to the lower animals, this curious relation of 

 sense-acuteness with colours may have had some influence on 



I Lorius, Eos (Trichoglossidsc), Eclectus (Palaeornithidae). 



Microglossus, Calyftorhynchus, Turacc^na. 

 3 Coracopsis, Alectranas. 

 * Medico-ChirurgLcil Transactions, vel. liii. (1870). 



the development of the higher human races. If light tints of 

 the skin were generally accompanied by some deficiency in the 

 senses of smell, hearing, and vision, the white could never com- 

 pete with the darker races, so long as man was in a very low or 

 savage condition, and wholly dependent for existence on the 

 acuteness of his senses. But as the mental faculties became 

 more fully developed and more important to his weKare than 

 mere sense-acuteness, the lighter tints of skin, and hair, and 

 eyes, would cease to be disadvantageous whenever they were 

 accompanied by superior brain-power. Such variations would 

 then be preserved ; and thus may have arisen the Xanthochroic 

 race of mankind, in which we find a high development of 

 intellect accompanied by a slight deficiency in the acuteness of 

 the senses as compared with the darker forms. 



I have now to ask your attention to a few remarks on the 

 peculiar relations of plants and insects as exhibited in islands. 



Ever since Mr. Darwin showed the immense importance of 

 insects in the fertilization of flowers, great attention has been 

 paid to the subject, and the relation of these two very different 

 classes of natural objects has been found to be more universal 

 and more complex than could have been anticipated. Whole 

 genera and families of plants have been so modified, as first to 

 attract and then to be fertilized by, certain groups of insects, 

 and this special adaptation seems in many cases to have deter- 

 mined the more or less wide range of the plants in question. It 

 is al-o known that some species of plants can be fertilized only 

 by particular species of insects, and the absence of these from 

 any locality would necessarily prevent the continued existence of 

 the plant in that area. Here, I believe, will be found the clue 

 to much of the peculiarity of the floras of oceanic islands, since 

 the methods by which these have been stocked with plants and 

 insects will be often quite difterent. Many seeds are, no 

 doubt, carried by oceanic currents, others probably by aquatic 

 birds. Mr. H. N. Moseley informs me that the albatrosses, 

 gulls, puffins, tropic birds, and many others, nest inland, 

 often amidst dense vegetation, and he believes they often carry 

 seeds, attached to their feathers, from island to island for great 

 di-tances. In the tropics they often nest on the mountains far 

 inland, and may thus aid in the distribution even of mountain 

 plants. Insects, on the other ban), are mostly conveyed by 

 aerial currents, especially by violent gales ; and it may thus 

 often happen that totally unrelated plants and insects may 

 be brought together, in which case the former must often 

 perish for want of suitable insects to fertilise them. This will, 

 I think, account for the strangely fragmentary nature of these in- 

 sular floras, and the great differences that often exist between 

 those which are situated in the same ocean, as well as for the 

 preponderance of certain orders and genera. In Mr. Pickering's 

 valuable work on the Geographical Distribution of Animals and 

 Plants, he gives a list of no less than sixty-six natural orders of 

 plants unexpectedly absent from Tahiti, or which occur in many 

 of the surrounding lands, some being abundant in other islands 

 — as the Labiat8e at the Sandwich Islands. In these latter 

 islands the flora is much richer, yet a large number of families 

 which abound in other parts of Polynesia are totally wanting. 

 Now much of the poverty and exceptional distribution of the 

 plants of these islands is probably due to the great scarcity of 

 flower-frequenting insects. Lepidoptera and Hymenoptera are 

 exceedingly scarce in the eastern islands of the Pacific, and it is 

 almost certain that many plants which require these insects for 

 their fertilization have been thereby prevented from establishing 

 themselves. In the Western islands, such as the Fijis, several 

 species of butterflies occur in tolerable abundance, and no doubt 

 some flower-haunting Hymenoptera accompany them, and in 

 these islands the flora appears to be much more varied, and 

 especially to be characterized by a much greater variety of showy 

 flowers, as may be seen by examining the plates of Dr. Seeman's 

 " Flora Vitiensis." 



Darwin and Pickering both speak of the great preponderance 

 of ferns at Tahiti, and Mr. Moseley, who spent several days in the 

 interior of the island, informs me that "at an elevation of from 

 2,000 to 3,000 feet the dense vegetation is composed almost 

 entirely of ferns. A tree-fern (Alsophila tahitensis) forms a sort 

 of forest, to the exclusion of almost every other tree, and, with 

 huge plants of two other ferns i^Angiopt.ris evecta and Aspelenium 

 nidus), forms the main mass of the vegetation " And he adds, 

 " I have nowhere seen ferns in so great proportionate abundance." 

 This unusual proportion of ferns is a general feature of insular 

 as compared with continental floras ; but it has, I believe, been 

 generally attributed to favourable conditions, especially to equable 



