Xll FLORA OF TASMANIA. 



manifested; but there is always a morphological change if the change of conditions be sudden, or 

 when, through lapse of time, it becomes extreme. The new form is necessarily that best suited to 

 the changed condition, and as its progeny are henceforth additional enemies to the old, they will 

 eventually tend to replace their parent form in the same locality. Further, a greater proportion 

 of the seeds and young of the old will annually be destroyed tban of the new, and the survivors of 

 the old, being less well adapted to the locality, will yield less seed, and hence have fewer descendants. 



In the above operations Nature acts slowly on all organisms, but man does so rapidly on the 

 few he cultivates or domesticates ; he selects an organism suited to his own locality, and by so modi- 

 fying its surrounding conditions that the food and space that were the share of others falls to it, he 

 ensures a perpetuation of his variety, and a multiplication of its individuals, by means of the destruc- 

 tion of the previous inhabitants of the same locality; and in every instance, where he has worked 

 long enough, he finds that changes of form have resulted far greater than would suffice to constitute 

 conventional species amongst organisms in a state of nature, and he keeps them distinct by maintain- 

 ing these conditions. 



Mr. Darwin adduces another principle in action amongst living organisms as playing an impor- 

 tant part in the origin of species, viz. that the same spot will support most life when peopled with 

 very diverse forms, as is exemplified by the fact that in all isolated areas the number of Classes, Orders, 

 and Genera is very large in proportion to that of Species. 



§ 3. 



On the General Phenomena of Distribution in Area. 



Turning now to another class of facts, those that refer to the distribution of plants on the sur- 

 face of the globe, the following are the most obvious: — 



14. The most prominent feature in distribution is that circumscription of the area of species, 

 which so forcibly suggests the hypothesis that all the individuals of each species have sprung froM a 

 common parent, and have spread in various directions from it. It is true that the area of some 

 (especially Cryptogamic and Aquatic plants) is so great that we cannot indicate any apparent centre 

 of diffusion, and that others are so sporadic that they appear to have had many such centres ; but 

 these species, though more numerous than is usually supposed, are few in comparison with those 

 that have a definite or circumscribed area. 



With respect to this limitation in area,* species do not essentially differ from varieties on the 

 one hand, or from genera and higher groups on the other ; and indeed, in respect of distribution, 

 they hold an exactly intermediate position between them, varieties *being more restricted in locality 

 than species, and these again more than genera. 



* It is a remarkable fact that there are some striking anomalies in the distribution of plants into provinces, as 

 compared with animals. Thns there is no peculiarity hi the vegetation of Australia to be compared with the 

 rarity of placental mammals, nor with the fact of so many of the mammals, buds, and fish of Tasmania differing from 

 those of the continent of Australia. Nearer home, we find the basin of the Mediterranean with a tolerably uni- 

 form Flora on the European and North African sides, but these ranking as different zoological provinces. The 

 much narrower delimitation in area of animals than plants, and greater restriction of Faunas than Floras, should 

 lead us to anticipate that plant types are, geologically speaking, more ancient and permanent than the higher 

 animal types are, and so I believe them to be, and I would extend the doctrine even to plants of highly complex 

 structure. 



