South America. The essay under
this title dealt with the subject in a most fascinating manner, and was
probably the first to arouse widespread interest in this aspect of
natural science.

The next eight years saw the production of many important and valuable
works, amongst which the "Geographical Distribution of Animals" (1876)
occupies the chief place. This work, though perhaps the least known to
the average reader, was considered by Wallace to be the most important
scientific work he ever attempted. From references in letters written
during his stay in the Malay Archipelago, it is clear that the subject
had a strong attraction for him, and formed a special branch of study
and observation many years before he began to work it out systematically
in writing. His decision to write the book was the outcome of a
suggestion made to him by Prof. A. Newton and Dr. Sclater about 1872. In
addition to having already expressed his general views on this subject
in various papers and articles, he had, after careful consideration,
come to adopt Dr. Sclater's division of the earth's surface into six
great zoological regions, which he found equally applicable to birds,
mammalia, reptiles, and other great divisions; while at the same time
it helped to explain the apparent contradictions in the distribution of
land animals. Some years later he wrote:

In whatever work I have done I have always aimed at systematic
arrangement and uniformity of treatment throughout. But here the immense
extent of the subject, the overwhelming mass of detail, and above all
the excessive diversities in the amount of knowledge of the different
classes of animals, rendered it quite impossible to treat all alike. My
preliminary studies had already satisfied me that it was quite useless
to attempt to found any conclusions on those groups which were
comparatively little known, either as regards the proportion of species
collected and described, or as regards their systematic classification.
It was also clear that as the present distribution of animals is
necessarily due to their past distribution, the greatest importance must
be given to those groups whose fossil remains in the more recent strata
are the most abundant and the best known. These considerations led me to
limit my work in its detailed systematic groundwork, and study of the
principles and law of distribution, to the mammalia and birds, and to
apply the principles thus arrived at to an explanation o