DARWIN'S LIFE. 133 
tion. It is well known that the islands in the South Seas 
consist for the most part of coral reefs, and are surrounded 
by them. Formerly no satisfactory explanation could be 
given of their different and remarkable forms, and of their 
relation to those islands which are not formed of corals. 
It was reserved for Darwin to solve this difficult problem, 
for together with the constructive action of the coral 
zoophytes, he assumed geological risings and depressions 
of the bottom of the sea to account for the origin of 
the different forms of reefs. Darwin’s Theory of the 
Origin of Coral Reefs, like his later one as to the Origin of 
Organic Species, is a theory which fully explains the 
phenomenon, and for this purpose assumes only the simplest 
natural causes, without hypothetically supporting it with 
any unknown processes. Among the remaining works of 
Darwin, I must not pass over his excellent monograph on 
Cirrhipedia, a curious class of marine animals, which in 
their outward appearance resemble mussels, and were 
actually considered by Cuvier as Molluscs possessing two 
shells, while in truth they belonged to the Crustacea (crabs). 
The extraordinary hardships to which Darwin had been 
exposed during his voyage in the Beagle had injured his 
health to such a degree, that after his return home he was 
obliged to withdraw from the restless turmoil of London life, 
and since then has lived in quiet retirement on his estate at 
Down, near Bromley, in Kent. This seclusion from the rest- 
less activity of the great city certainly exercised a beneficial 
influence upon Darwin, and it is probable that we owe to it, 
at least partially, the formation of the Theory of Selection. 
Undisturbed by the various engagements which in London 
would have wasted his strength, he was enabled to concen- 
