772 Working v. Fighting . [December, 
they have taken their rise, cannot possibly explain their first 
appearance. Hence, before we can account for the origin of 
species we must know whether such conditions are in any 
case found to set up a tendency to variation, of which selec- 
tion may take hold. 
Mr. Wallace treats of the origin, and especially the dis- 
persal, of species from a different point of view, basing his 
considerations on the faunas and floras of islands. He 
classes islands as oceanic — those formed by volcanic or 
coralline adtion in the midst of wide seas and as conti- 
nental, i.e., such as have evidently at one or other point of 
time formed part and parcel of some adjacent continent. 
This latter group he sub-divides according as their separa- 
tion from the mainland is of recent or of ancient date. 1 he 
three classes thus formed present respectively most remark- 
able differences in their animal and vegetable population, 
and in pointing out these distinctions and explaining how 
such regions have first received their inmates, he is led to 
conclusions which may be usefully compared with those of 
Prof. Semper. Both authors examine the influence _ of 
temperature and recognise the changes of climate which 
have undoubtedly occurred upon our earth as important 
factors in the origin of species. Mr. Wallace shows, how- 
ever, that “ we find no indication that the almost perfect 
similarity of climate and general conditions has any tendency 
to produce similarity in the animal world.” Prof. Semper, 
who has conducted very important experiments on certain 
species of mollusks exposed to abnormal temperatures, comes 
to the kindred conclusion that the fossil fauna of any region 
affords no trustworthy evidence as to its climate at any 
particular past epoch. He brings forward the remarkable 
faCt that parrots, distinctly tropical birds, accustomed to an 
average temperature of 8o° Fahr., have been found capable of 
living and increasing in the open-air in England with a 
mean temperature not exceeding 50° Fahr. 
Such considerations ought to inspire geological speculators 
with a wholesome caution. 
Mr. Wallace enters at length into the aaion of oceanic 
currents and of storms in the distribution of species, and by 
their means he explains, as it seems to me very happily, the 
peculiarities of the faunas of the Azores, the Bermudas, the 
Gallapagos, and other remote island groups. 
Professor Semper, too, examines very fully the agency of 
currents both as promoting and as limiting the distribution 
of animals, and as sometimes checking the. formation of 
new species. As regards the aaion of the wind in regulating 
