434 PROFESSOR LANKESTER. 



parts of the organism has been in this way slowly built up. 

 The final advantage of highly complicated structure has been 

 gained by certain representatives of the animal and the vege- 

 table pedigree through Adaptation, due to the property of 

 unlimited variation, and through Heredity, which is only 

 another name for Memory or permanence of impression, as 

 manifested by the detached reproductive bits of an organism. 



9. The process of the development of new tissues or new 

 organs in a race of organisms where such tissue or organ 

 has no previous existence, must follow certain definite 

 methods. From the nature of the causes at work (sees. 7 

 and 8), the new developmeilt; must be excessively gradual. 

 It may be taken as a law of development that no really 

 new part ever does make its appearance, every apparently 

 new tissue or organ which may strike the morphologist as 

 novel, being necessaiily only a modification of a pre-existing 

 tissue or organ. The absolute continuity of forms is a 

 deduction from the law of evolution, and the hypothesis of 

 unity of organization. 



The processes of differentiation by which organisms acquire 

 modifications in structure, may be grouped under the follow- 

 ing general heads : (1) Polar Repetition of units of structure. 

 (2) Segregation of chemically and physically differing mate- 

 rials. (3) Hypertrophy and Atrophy of parts relatively to 

 one another. (4) Concrescence (of polar units or of appen- 

 dages and tissues). 



Polar Hepetition stands first in this list, since it is 

 dependent on one of the most important and distinctive 

 features of protoplasm, with which only crystalline polarity 

 can be compared. The existence of multicellular organisms 

 instead of large unicellular organisms is due to the peculiar 

 conditions of molecular cohesion in protoplasm ; in iact, the 

 polar repetition of the simplest organic unit is at the bottom 

 of all the higher organic diff"erentiation. Further, we find 

 that the groups formed by the primary units or plastids, 

 and which Herbert Spencer calls " aggregates of the second 

 order," may, like the primary units, cease to grow indefi- 

 nitely as secondary units, and, with or without fission, the 

 growing secondary unit arranges itself as a nutnbcr of such 

 secondary units (in line as in worms, or irregularly as in 

 polyp-trees, or radially as in compound Tunicates), and 

 itself becomes " an aggregate of the third order." 



Thus the polarity of protoplasm is a very important 

 element in the differentiation of organic forms. 



Segregatio7i may slowly bring about a diflference in the 

 substance of two portions of the same unit, and at last estab- 



