an important kind, bearing on the general problems that even specialists have to follow, they all at once set to work in their laboratories to make corroborative dissections or experiments, and it is part of every modern account of a biological discovery to tell exactly the methods by which results were got, in order that this process of corroboration may be set about easily.  The question as to whether or no natural selection were the sole or chief cause, or indeed a cause at all, of evolution is not yet, and perhaps never will be, a matter of knowledge in the scientific sense.  At the most, we can see for ourselves only that selection does bring about changes at least as great as the differences between natural species.  The evidence for this we have before our eyes, if we choose to see, on a stock farm; in the breeding yards of any keeper of “fancy” animals; or in the nursery gardens of any florist.  So far, Huxley accepted the Darwinian principle as a definite contribution to knowledge; and so far the whole body of biologists has followed him.  Beyond this the truth of the Darwinian principle is a matter of inference or judgment; of balancing probabilities and improbabilities.  In multitude of counsellors there is said to be wisdom, and what we learn from the counsellors of biology all over the world is that some maintain that natural selection is the only probable agency in effecting evolution, and that it is competent to account for all the changes which we know to have taken place; others hold that its probable influence has been over-rated; and others, again, think that it has been one of the many causes that have brought about the kaleidoscopic variety of organic nature.  Huxley remained to the last among those who distinguished in the clearest way between natural selection as an exceedingly ingenious and probable hypothesis, and a proved cause; and he was always careful, especially when he was writing for or speaking in the presence of those who like himself accepted the fact of evolution as proven, to distinguish between this provisional hypothesis as to how evolution had come about, and definite knowledge that it had come about in this way.  Two passages from Huxley’s writings, one written in 1860 in the Westminster Review, and the second written in 1893, in the preface to the volume of his collected essays which contained a reprint of the Westminster article, will make plain the continuity of Huxley’s attitude: 

“There is no fault to be found with Mr. Darwin’s method, then; but it is another question whether he has fulfilled all the conditions imposed by that method.  Is it satisfactorily proved, in fact, that species may be originated by selection?  That there is such a thing as natural selection?  That none of the phenomena exhibited by species are inconsistent with the origin of species in this way?  If these questions can be answered in the affirmative, Mr. Darwin’s view steps out of the rank of hypotheses