HEEBEET SPENCEE's BIOLOGY. 375 



rather than another, and to assume cei'tain forms in some instances, 

 certain other forms in other cases. On the latter points Mr. 

 Spencer does not dilate ; he merely states that the factors bringing 

 about the changes of form in plants are external, incident forces, or 

 internal, hereditary proclivities, so that " every organism is the re- 

 sult of a compromise between internal forces tending to reproduce 

 ancestral forms, which were in harmony with incident forces at the 

 time of their evolution, and external incident forces tending to cause 

 deviations from that form." 



The two processes of growth and development, and the agencies 

 influencing them, being thus alluded to, our author then proceeds to 

 seek for the physiological units out of which the organisms are to 

 be built. These are specks of protoplasm manifesting life, and yet 

 shoAving no sign of organization. Organic evolution begins by the 

 formation of a minute aggregate of them into a cell. We may expect 

 *' that as structureless portions of protoplasm must have preceded 

 cells iu the process of general evolution, so, in the special evolution 

 of each higher organism, there will be an habitual production of 

 cells out of structureless blastema ;" and moreover, " that tissue may 

 occasionally be formed by the direct transformation of the blas- 

 tema." We regret that Mr. Spencer has not pursued this portion 

 of his subject further, and shown us what tissue he expects to be so 

 formed, where to look for it, how to recognize it. Is it the struc- 

 tureless cuticular layer overlying the epidermis of flowering plants, 

 or the gelatinous envelope of such plants as Fahnella, or the mem- 

 brane lining the air-canals of Victoria ? But on this point we are 

 left in the dark. The void is in part supplied by the writings of 

 Drs. Lionel Beale and Hughes Bennett on organic molecules, and 

 on the structure and growth of cells. 



A cell, then, is an aggregate of the first order, formed of physio- 

 logical units united into a group, that is structurally single, and 

 cannot be divided without destruction of its individuality. Of such 

 cells we have numerous examples in the lower AlgsB, Fungi, &c., the 

 cells being sometimes of considerable size and much branched. 

 Aggregates of the second order are plants composed of more than 

 one cell, the cells being sometimes held together by a matrix of 

 mucus, and aggregated in one plane, either in linear series, or 

 radiating from a centre. In other cases they may be grouped on 

 all sides of a central point, and so form a spherical mass. It is 

 not necessary to follow Mr. Spencer in his review of the filamentous, 



