clxii Introduction. 



soria ; but the ' polymorphismus' of tlieir more complex forms, amongst 

 whicli the Radlolaria are usually included, may be considered in some 

 sense to counterbalance the higher grade of specialization to which the 

 Infusoria in virtue of their digestive, reproductive, and motor organs, 

 must be allowed to have attained. The Spongiadae should, for the same 

 reason and in the same sense as the Ehizopoda, be placed in co-ordinate 

 rank with the Infusoria. 



It is not rarely difficult to differentiate a unicellular organism as animal 

 or vegetable, unless we happen to be acquainted with its past or future 

 history. The gonidia of many Algae are locomotor, ciliated, possessed of 

 contractile vesicles, and devoid, whilst yet active, of that eminently vege- 

 table structure, an encapsulating envelope of cellulose. Whilst these 

 organisms imitate the movements, structure, and chemical composition 

 of the Infusorial Protozoa, the mycelium of certain Fungi, the Myxo- 

 gastres, s. Myxomycetes, which have hence been called ' Mycetozoa,' is 

 similarly devoid of any cellulose envelope, and exhibits the pseudopodial 

 movement so char-acteristic of Rhizopoda ; while such forms of life as 

 Euglena Viridis and the Volvocineae must be held to belong to the 

 vegetable kingdom, not so much on account of the abundance of chloro- 

 phyll in their parenchyma, as because of the histoiy of their develop- 

 ment, in which a period of quiescence and encapsulation in cellulose is 

 readily observable. 



There are not a few organisms for the identification of which, as 

 belonging to the animal or vegetable kingdom, we have no other guide 

 than a consideration of their gradational affinities to other organisms, as 

 to the position of which in one or other of the two kingdoms there can 

 be no question. By the application of this test, it would seem that the 

 Monera of Professor Haeckel should be ranked as animals, as they are so 

 closely similar to certain of the Ehizopoda, as to the right of which Class 

 to be considered as animal few other naturalists would raise a doubt. 

 And as either from the point of view furnished by the facts of gradational 

 affinity, or from that into which we are put by the knowledge of the 

 history of development, probably all the other forms of life out of which 

 tlie naturalist just mentioned has formed a third Kingdom of life, the 

 Eegnum Protisticum, can, without violence, be regarded as either animal 

 or vegetable, the necessity for accejiting such a third Kingdom would 

 appear to be doubtful. 



In dealing Avith the microscopic organisms, about the position of which 

 it is possible to raise a doubt, the most unambiguous criterion is that 

 which the formation of an external envelope of cellulose, when present, 

 furnishes. When this means of deciding is absent, the exhalation of 



