In their onward development from the lowest forms of life, man and the higher animals have not only permanently retained in their bodies certain parts which resemble organisms low in the scale of existence, but every now and again a tendency to reversion appears in certain individuals, and we thus get anatomical abnormalities and malformations.

These were formerly inexplicable, but the doctrine of evolution has thrown much light on their probable causation.

Now and again we also meet with peculiarities in the reaction between drugs and parts of the human body in certain individuals.

Some persons, for example, are like pigeons - only slightly affected by opium - and can take enormous doses of it without any apparent effect. Others, again, are peculiarly sensitive to the action of certain medicines, and a dose of a mercurial preparation, which would have but a slight purgative action on one, will produce intense salivation in another.

These personal peculiarities in regard to the action of drugs, or idiosyncrasies, as they are termed, have been, and are still, very perplexing to the medical practitioner. It is probable, however, that a more complete study of comparative pharmacology will enable us, to some extent at least, to recognise these, and thus to avoid the inconvenience which they occasion.