And then we have to add to these enumerations of living species the extinct species of successive geological ages, the remains of which are sufficiently well preserved to admit of identification. Those which are known are only a few thousands in number, and a mere fragment of the vast series of species which _have_ existed in successive past ages of the earth. They are a few samples of the predecessors of the existing species, and some of them were the actual ancestors of those existing to-day. The larger number of them have left no direct issue, but represent side branches of the "tree of life" which have died out ages ago.
[Illustration: STRANGELY-SHAPED FISHES.–1. The Coffer-fish (Ostracion); 2. Pteraclis, an oceanic fish allied to the so-called Dolphins; 3. The Sun-fish (Orthagoriscus); 4. An Australian Blenny Patæcus.]
The subject treated in this and the next chapter is one of the most interesting to mankind, and is surrounded by extraordinary prejudice, sentiment, and ignorance. It is one upon which really trustworthy information is to a very large extent absent–and difficult to obtain. I cannot profess to supply this deficiency, but I can put the matter before the reader.
It is a well-established fact that the various "kinds" of animals and of plants do not breed promiscuously with one another. The individuals of a "species" only breed with other individuals of that "species." They do not even, as a habit, breed with the individuals of an allied species. So nearly universal is this rule that it was for a long time held by naturalists to be an absolute definition of "a species," that it is a group of individuals capable of producing fertile young by breeding with one another and incapable of producing fertile young by mating with individuals of another such group, which were, therefore, held to constitute a distinct species. The practical importance of this definition was that it could, in a large number of instances among animals, and still more amongst plants, be made use of as a test and decided by experiment.
It is a curious fact that popular belief amongst country-folk and those who have opportunities of coming to a conclusion on so simple and direct a question has never accepted this law of the limitation of species in breeding as more than a general rule to which it has always been supposed that frequent exceptions occur. I mention this not in order to add that "there is always some basis of truth in these popular beliefs," but on the contrary to point out that popular beliefs on such matters are very frequently altogether erroneous, and though their origin can sometimes be explained, it is rare to find that they are due, in however small a degree, to true observation and inference. Where the subject under consideration has the obscurity and strong fascination for the natural man which all that relates to the processes of life, growth, and reproduction possess, we find that traditional fancies of the most unwarrantable kind are current, and hold their ground with tenacity even at the present day. Some 250 years ago, and earlier–in fact, before the commencement of that definite epoch of "the New Philosophy" marked by the foundation of the Royal Society of London–any queer-looking animal brought from remote lands, and any misshapen monstrosity born of cattle, sheep, dogs, or men, was "explained," and confidently regarded as a "hybrid," the result of a "cross" or irregular coupling of two distinct species of animals to which the "monster" presented some fanciful resemblance. Whole books were devoted to the description and picturing of such supposed examples of mis-begotten progeny.