In 1880, Mr. A. W. Bennett, in a paper read at the Swansea meeting of the British Association, proposed the following classification:—

Thallophytes.

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Then he proposes to subdivide the Zygophyceæ, the Oophyceæ, and the Carpophyceæ thus:—

The earlier classifications need not be considered, as they will doubtless be superseded by those of Sachs and Bennett. But even the systems of these two last mentioned botanists appear to me to be open to several objections.

(1st.) By both, the Phæosporeæ are placed amongst Oosporeæ, though probably nearly all of them are reproduced by conjugation. (2nd.) Again, by both the Hydrodictyeæ are separated from their nearest allies, the Volvocineæ. (3rd.) Sachs places the Botrydiaceæ under the Oosporeæ, but conjugation alone is known in them; while Mr. Bennett separates them widely from the Siphoneæ, which are undeniably their nearest relations.

(4th.) The Confervaceæ are widely separated from the Sphæropleæ by both Sachs and Bennett; but these two orders differ only in their mode of reproduction, whilst they closely resemble each other in general structure and appearance.

It seems to me that the principal cause of error in these two classifications is the idea that all the Oosporeæ must be closely related, for behind this idea is the belief that fertilization has arisen only once in the vegetable kingdom; that is to say, that all plants which are reproduced by fertilization are descended from a common stock. But this scarcely appears to me to be correct, for fertilization has evidently arisen independently in the animal and vegetable kingdoms, as it is not found in the Protozoa, or the Palmellaceæ, the lowest divisions of each kingdom. Again, looking at the Algæ themselves, we see that fertilization is not the same process in all. The oospheres of Fucus, for example, differ considerably from the oospheres of Vaucheria; but, at the same time, the structure of the stem in the two genera is totally dissimilar. Now, it is much easier to suppose that fertilization has arisen independently in these two groups, than that they have diverged from a common ancestor, reproducing itself in this way. For it can scarcely be doubted that fertilization first originated in conjugating zoospores. We have only to suppose (and the supposition is perfectly warrantable) that an advantage was gained by a specialization of the functions of the two cells; the one increasing in size and becoming passive, the other remaining small and motile, since it would have to penetrate into the interior of its companion cell. Through some such variation as this, fertilization might easily have arisen on various occasions.

One more objection might be taken to Sachs' classification of the Algæ. He has included the Characeæ under the Carposporeæ; though it appears probable that they should stand by themselves. Perhaps they are degenerate forms of a higher type; for it is only in habit and, to some extent, in structure of stem, that they at all resemble the Algæ; and we know that many freshwater plants have become much simplified in structure (e.g. Marsilia and many of the Naiadeæ). A plant living in water has no need for a stem built up of many tissues. The simplicity of the stem of Chara does not necessarily therefore connect it with the Algæ, and its reproduction, on the other hand, can scarcely be compared with that of the Florideæ and Coleochæteœ. Professor Sachs has endeavoured to trace out homologies between the two, which, however, to me appear to be far-fetched and doubtful. It is much easier to suppose that the Characeæ is an order standing by itself, than to consider it as allied either to the Algæ or the Musci. Mr. Bennett has well dealt with this question in the “Journal of Botany,” 1878, p. 202; so it will not be necessary to go into any detail here.

As a lineal classification of the Thallophytes is impossible, I would tabulate them thus, the orders of the Algæ being given in full:—

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The genealogical tree on Plate Xa. seems to me to show, as nearly as our present knowledge will permit, the genetic affinities of the different orders of the Algæ. Of course, much of it is provisional and somewhat doubtful. For example, I have placed the Palmellaceæ as the lowest group, but this position may belong