As a young man, Wallace taught in the Collegiate School, Leicester.  While in Leicester, a chance meeting with Henry Walter Bates in the public library led to a close friendship, and to their plan for a joint expedition to the Brazilian Amazon.

Wallace on species:

"In estimating these numbers [i.e. of species in different regions] I have had the usual difficulty to encounter, of determining what to consider species and what varieties. ... The rule, therefore, I have endeavoured to adopt is, that when the difference between two forms inhabiting separate areas seems quite constant, when it can be defined in words, and when it is not confined to a single peculiarity only, I have considered such forms to be species. When, however, the individuals of each locality vary among themselves, so as to cause the distinctions between the two forms to become inconsiderable and indefinite, or where the differences, though constant, are confined to one particular only, such as size, tint, or a single point of difference in marking or in outline, I class one of the forms as a variety of the other."

"Species are merely those strongly marked races or local forms which, when in contact, do not intermix, and when inhabiting distinct areas are generally regarded to have had a separate origin, and to be incapable of producing a fertile hybrid offspring.  But as the test of hybridity cannot be applied in one case in ten thousand, and even if it could be applied, would prove nothing, since it is founded on an assumption of the very question to be decided - and as the test of origin is in every case inapplicable - and as, further, the test of non-intermixture is useless, except in those rare cases where the most closely allied species are found inhabiting the same area, it will be evident that we have no means whatever of distinguishing so-called "true species" from the several modes of variation here pointed out, and into which they so often pass by an insensible gradation."

From pp. 4 and 12 in: A.R. Wallace (1865).  On the phenomena of variation and geographical distribution as illustrated by the Papilionidae of the Malayan region. Transactions of the Linnean Society of London 25: 1-71. Today, the original is available online, at archive.org, and also various other editions, for instance the shorter version, as printed in Wallace's  book "Contributions to the Theory of Natural Selection" (1871), from the University of Michigan. See Mallet 2009 for a discussion of the importance of this paper for understanding species and speciation.

Wallace, of course, had independently discovered the principle of natural selection in 1858.  But he always revered Darwin, even though Darwin took most of the credit after his book "On the Origin of Species" (1859). Here is an interesting observation by Wallace, in 1809, on the occasion of Darwin's centenary:

"How strange it seems to us now, that, but for the fortunate chance of a naturalist being wanted just when Darwin had passed his examination, and professor Henslow being the person asked to recommend one, he would almost certainly have become a clergyman; in which case he would have been, no doubt, an admirable country parson, but the “Origin of Species” would not have been written..."

Bibliography
(there have been many biographies of Wallace; here I list a few of my favourite sources)

Fichman, M. (2004) An Elusive Victorian: The Evolution of Alfred Russel Wallace. University of Chicago Press, Chicago.

Knapp,S (1999): Footsteps in the Forest.  Alfred Russel Wallace in the Amazon. The Natural History Museum, London. vi+90 pages.

Shermer,M (2002): In Darwin's Shadow.  The Life and Scienceof Alfred Russel Wallace.  A Biographical Study on the Psychology of History. Oxford University Press, New York. xx + 422 pages.

Smith, C.H.: The Alfred Russel Wallace Page.  http://people.wku.edu/charles.smith/index1.htm