The geographical distribution of animals, with a study of the relations ...

462 ZOOLOGICAL GEOGRAPHY. [part in.all the great continents. That event certainly dates back toSecondary, if not to Palaeozoic, times, because so dominant agroup must soon have spread over the whole continuous landareaof the globe. We have no reason for believing that birdswere an earlier development ; and certainly cannot, with anyprobability, place the origin of the Struthiones before that ofMammals.Causes of the Poverty of Insect-life in New Zealand : its Influenceon the Character of the Flora.—The extreme paucity of insectsin New Zealand, to which we have already alluded, seemsto call for some attempt at explanation. No other country in the•world, in which the conditions are equally favourable for insectlife,and which has either been connected with, or is in proximityto, any of the large masses of land, presents a similar phenomenon.The only approach to it is in the Galapagos, and insome of the islands of the Pacific ; and in each of these cases theabsence of mammals leads us to infer, that no connection with acontinent has ever taken place. Yet the fauna of New Zealandevidently dates back to a remote geological epoch, and it seemsstrange that an abundance of indigenous insects have not beendeveloped, especially when we consider the vastantiquity thatmost of the orders and families, and many of the genera, of insectspossess (see p. 166), and that they must always have reached thecountry in greater numbers and variety than any of the higheranimals. The undoubted fact that such an indigenous insectfaunahas not arisen, would therefore lead us to conclude, thatinsects find the conditions requisite for their development only inthe great continental masses of land, instrict adaptation to, anddependance on, a varied fauna and flora of ever-increasing richnessand complexity. A small number of widely-separated forms, introducedinto a country where the fauna and flora are alike scantyand unrelated to them, seem to have little tendency to varyand branch out into that vast network of insect-life whichenriches all the great continents and their once connectedislands.It is a striking confirmation on a large scale,of Mr. Darwin'sbeautiful theory—that the gay colours of flowers have mostly, or