468 THE POPULAR SCIEXCE MOXTHLY. 



adhesion of surfaces confined to a single leaf, as in the iris. But these 

 adhesions are all topical, if I may use the word, and do not affect the 

 type and general axis. 



Now, this involution, by which the progress of evolution takes 

 place, may, as remarked, and strange to say, be described as a failure 

 of development — a failure to unfold into the original type, constituting, 

 by the very failure, a new and higher type. If we watch the develop- 

 ment of any leaf from venation, we observe that in some way it was 

 folded down upon itself, or rolled up with its fellows of the same bud, 

 and before coming to adult age it expanded into the recognized form 

 of its order. Let but that expansion fail, and the evolution may be 

 said to fail. A happy failure ; for to this we owe the production of 

 grain and fruit — the food of man. Adhesions take place within the 

 buds, which change the leaf-buds into flower-buds. The change is 

 always made in the embryo, while still plastic and capable of being 

 moulded into new forms. 



It would be delightful to follow the immortal Linnaeus, Wolf, 

 Goethe, and the grand army of enlightened living botanists, who have 

 illustrated this beautiful transcendental history of leaves, and flowers, 

 and fruits ; but time forbids at present. Sufiice it to say that, so far 

 as the vegetal kingdom is concerned, the doctrines of involution have 

 become the common property of the scientific world. 



Let us pass on to the so-called animal kingdom. 



And first, as to the bridge we cross. True, it is somewhat the 

 fashion to tell us that here is a great gulf fixed, and no crossing was 

 ever possible. Yet Nature found a bridge somewhere, and we ought 

 not to despair of finding at least some remains of the abutments. 



To illustrate this passage from the vegetal to the animal plan of 

 structure, take a hollow India-rubber ball, which may very well repre- 

 sent a cell enlarged a few thousand diameters. It is a perfect image of 

 an external living creature, and is also typical of one stage of the de- 

 velopment of the ova of the lower orders of animals — probably of 

 all animals. But it is the adult form of plants. The vital functions 

 of such a cell are all within ; and there is no communication with the 

 external world, except by osmotic action. Through this same cell- 

 wall — this same external coat — and by means of it, nutrition and aera- 

 tion are both carried on. This is an exothen. 



Now, by pressing the finger upon a part of the elastic coat, a j)or- 

 tion of it sinks in, and you have a cup — a cup with double walls and a 

 space between them. This is an involution ; and by means of such an 

 involution as this Nature transforms a plant into an animal. 



Recently-published observations of Kowalewsky declare that he 

 has seen this sort of transformation actually take place in the growth 

 of the embryo of a creature as high in the scale as the Ascidians. 

 Doubtless, in Nature, the true process was a failure at one point to fill 

 out the rounded fullness of the ball ; some contraction, some atrophy, 



