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In On the Origin of Species (1859), Charles Darwin explained how species originate via a continuous process of natural selection that shapes organisms, over eons of time, into the funtioning complexities that they are, suited for a particular way of life. In The Descent of Man (1871), Darwin applied his theory of evolution by natural selection to mankind. The difference between mankind and animals is one of degree, not of kind: there are vestiges in man of our animal past and there are primitive traces of man's capabilities in animals. The different races of mankind have come about -
according to Darwin, not to modern evolutinary biologists - through sexual selection.

During the writing of The Descent of Man, Darwin collected more and more information on the expression of emotions in mankind and in animals. He decided to cut it from manuscript and make it into a standalone book: The Expression of Emotions in Man and Animals (1872).

There is a clear gap between his Origins and Descent on the one side and Expression on the other side, though. The first two books are classics and can still be read by modern day readers: they contain gems of truth and are, historically speaking, influential masterpieces. Expressions is nothing like this. I will explain why this is so in following paragraphs, and this will let anyone decide is he/she wants to read this book or not.

Darwin's main thesis in Expressions is that the (for the most part involuntary) emotions that we express daily are the result of our evolutionary past. The expression of these emotions had an adaptive function during the lifetimes of our ancestors. This means (1) that animals and humans share common expressions (or at least the mechanisms of expressing those emotions) and (2) that human emotions have their origin in animal instincts.

The way Darwin works out his thesis is unorthodox. He compiles lots and lots of data and meticulously describes all the physical expressions of the emotions; each chapter deals with different types of emotions and therefore the Expressions can better be viewed as an 'Encyclopedia of Emotions' than as a book. This is one of the main reasons why Expressions is not really interesting for modern day readers: endless summations of emotions - not really an attractive foresight.

Another point that has to be stressed is that modern day cognitive psychology and neuroscience have unearthed much knowledge that Darwin didn't have access to. This means that Darwin's interpretations are almost all outdated. This makes it, sort of, redundant to read Expressions.

So why read it at all? Well, there are two important reasons for reading this.

First, one should focus on Darwin's method and not on his answers. Darwin tried to argue that human beings and animals share a common past and therefore share a lot of commonalities in the expression of emotions. Darwin collects evidence to prove his point via many different methods. He sent questionnaires abroad to the British colonial enterprises to acquire data on emotions from different races; he used the then-new technique of photographing people; he studied the insane (on the assumption that these people show pure emotions, whereas healthy persons cover up a lot of emotions); he used the research of Duchenne who used electroshocks to stimulate different facial areas to study the emotions; he observed children (mostly his own) in their emotive expressions; and he gathered much information of other (international) experts in the field.

In short, Darwin gathered an enormous database. That his analysis was qualitative (i.e. not statistical, as most of modern science is) is a minor detail: the content of his database was faulty, incomplete and he analyzed it in the wrong way - yet, his method was unique compared to his contemporaries. In other words, Darwin was the first scientist to study the emotive expressions of man and animals in a scientific way.

Second, Darwin's contemporaries used obscure hypotheses to explain all of mankind's peculiarities. Darwin was one of the few who was prepared to go the whole mile. As soon as you accept evolution by natural selection as the driving force behind natural diversity, mankind's favored position crumbles (hence The Descent (!) of Man). No longer is there a hierarchy in nature; all that remains is a tree of geneaology, of which mankind is just one little (and very recent) branch. In other words, mankind is a product of evolution, just like the rest of life on planet Earth, and hence we have to explain mankind's features with reference to our evolutionary past. This last step was too much for most of Darwin's contemporaries (Wallace, Lyell, etc.), who tried to come up with pseudo-scientific and arbitrary reasons why mankind was to be regarded as special creation (for example, clinging on to Descartes' dualism of mind/body).

Darwin tried in his Descent and Expressions to come up with a uniform and general principle, a natural explanation, for all of mankind's characteristics and traits. In that sense, Expressions is 'just' volume 3 of Descent. It is important to realize what Darwin did: for the first time a scientist offered a (convincing) natural explanation for all that man is. Mankind differs in degree from the other animals; there is no distinction in kind between man and the rest of life. This message is not really explicitly stated in Darwin's own work, but one can see with hindsight Darwin's revolutionary break with the past. So, this is a good reason for reading Expressions (especially if one has read, and enjoyed, The Descent of Man).

So, to sum up: the content of Expressions is outdated, its method is flawed, yet the revolutionary character of offering a natural explanation for the emotions of man (and animals) is a timeless component of this book. It is a historical document, and offers a fruitful way of thinking about humanity.

As a last remark, I'd like to add that Darwin's approach to the psychology of mankind (in Expressions our emotional sphere) has been vindicated in the (fairly recently established) field of evolutionary psychology. This science tries to explain all of man's mental faculties by tracing them to our evolutionary past. This has led to a whole new way of interpreting the results of mainstream psychology, which offers only proximate explanations, whereas evolutionary psychology offers ultimate explanations. This fact alone establishes the wisdom of Charles Darwin, who has been vindicated on lots of different points (his theory of evolution by natural selection, the role of sexual selection, etc.). The man clearly was a genius.