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To prove that the Darwinian theory (1) is a good working hypothesis, it is only necessary to point to the fact that every great advance made during late years in biological science has confessedly been the result of investigations carried out more or less under its influence; nor is this to be wondered at any more than that astronomy should have received an impetus from the discovery of the telescope or the study of minute life from the application of the microscope, this theory being simply an instrument by means of which scientific men are better able than any former one to explain large and exceedingly diverse groups of facts.

No one has made more use of this instrument than Darwin himself, whose biological works now form a large series. The work before us is the second from his pen during less than a year, and, like, its predecessor, it shows no diminution in the author's industry in collecting facts, nor in his unrivalled power of marshalling these so as to render them intelligible.

His hypothesis is applicable to all life, whether found in the trunk of a tree or in the body of an animal, and his investigations have led him as often into the vegetable as into the animal kingdom, the results obtained in the former being equally curious and suggestive, although lacking somewhat in the personal interest attaching to those drawn from the latter. The difference in the forms of flowers on plants of the same species is mainly dependent on differences in their sexual relations, these being much more complicated in plants than in animals; thus each flower, in many species of plants, is both male and female, or hermaphrodite, as it is called; in some it is either male or female, both sexes occurring on the same plant; in others the flowers are all male on one individual of a species, and all female on another of the same species, while there is still another group, known as polygamus, in which all the above forms occur; thus in 15 ash trees growing in the same field, Darwin found that 8 produced male flowers, while 3 were hermaphrodites, the last having a different aspect when in flower from the others.

Again, in many genera of plants the inflorescence consists of three kinds of flowers—the outer ones large and conspicuous but sterile, those immediately behind smaller but open, fairly sterile, and capable of cross-fertilisation, with the centre flowers still smaller, entirely closed, exceedingly fertile, and only capable of self-fertilisation. In such cases the outer flowers, according to Darwin, are conspicuous in order to attract insects whose visits are necessary to the fertilisation of the inner flowers.

What those varieties in the forms of flowers are, the purposes they serve, and how they may have been produced, are the topics chiefly discussed in this volume, and these are treated with a wealth of information which requires in the reader only a moderate acquaintance with the elements of botany in order fully to understand them.

The chapters dealing with the heterostyled forms of hermaphrodite species, which Darwin was the first to observe and describe many years ago, are specially full and clear, and their perusal must fill the reader with astonishment at the wonderful diversity of the contrivances provided to ensure the cross fertilisation of plants. These heterostyled forms have also supplied the author with an answer to those who maintain the fixity of species merely because the offspring of a cross between distinct species are barren, for in these flowers the sexual elements of the same species may come together in a great many ways, all of which would be sufficient to produce fertilisation in animals, but it is only in a few of these, known as legitimate union, that the seeds are fertile.

"Naturalists," says Darwin, "are so much accustomed to behold great diversities of structure associated with the two sexes, they feel no surprise at almost any amount of difference; but differences in sexual nature have been thought to be the very touchstone of specific distinction.

We now see that such sexual differences—the greater or less power of the fertilising and being fertilised—may characterise the co-existing individuals of the same species, in the same manner as they characterise, and have kept separate, those groups of individuals produced during the lapse of ages, which we rank and dominate as distinct species.