The subject on generation is not only interesting as a branch of science, but it is so connected with the happiness of mankind that it is highly important in a practical point of view. Such, to be sure, is the custom of the age, that it is not considered a proper subject to investigate before a popular assembly, nor is it proper to attend the calls of nature in a like place; yet they must and ought to be attended to, for the good, the happiness of mankind require it; so, too, for like reason, the subject of generation ought to be investigated until it be rightly understood by all people, but at such opportunities as the good sense of every individual will easily decide to be proper. This, I presume to say, not simply upon the abstract principle that all knowledge of Nature's workings is useful, and the want of it disadvantageous, but from the known moral fact that ignorance of this process has in many instances proved the cause of a lamentable "mishap," and more especially as it is essential to the attainment of the great advantage, which it is the chief object of this work to bestow upon mankind. People generally, as it was the case with physicians until late years, entertain a very erroneous idea of what takes place in the conception. Agreeably to this idea the "check" which I consider far preferable to any other, would not be effectual, as would be obvious to all. Consequently, entertaining this idea, people would not have due confidence in it. Hence, it is necessary to correct a long-held and widely extended error. But this I cannot expect to do by simply saying it is an error. Deeply rooted and hitherto undisputed opinions are not so easily eradicated. If I would convince any one that the steps in one of the most recondite processes of nature are not such as he has always believed, it will greatly serve my purpose to show what these steps are. I must first prepare him to be reasoned with, and then reason the matter all over with him. I must point out the facts which disprove his opinion, and show that my own is unattended with difficulties. But what can be more obvious than that it is absolutely impossible to explain any process or function of the animal economy, so as to be understood, before the names of the organs which perform this function have been defined, that is, before the organs themselves have been described. Now it is well known to every anatomist, and indeed it may be obvious to all, that in describing any organ or system of organs, we must always begin with some external and known parts, and proceed regularly, step by step, to the internal and unknown. As in arithmetic, "everything must be understood as you go along."* * This is an Americanism, which appears to us to convey a false idea. If it refers to the cases used as illustrations, Dr. Knowlton is more sparing in his use of them than either Dr. Bull or Dr. Chavasse.--Publishers' Note.