No. 596] 



EVOLUTIONAL 1 V TH EOB V 



453 



changes receives support from various collateral fields 

 of investigation. 



1. First, from embryology, by analogy. The develop- 

 ment of the embryo is directed from within. The process 

 of development is one of specialization; a great number 

 of tissues is produced but these tissues have lost the 

 capacity, which the embryo has, of producing all kinds 

 of tissues. Development is essentially an irreversible 

 process just as evolution is, and for the same reason- 

 that a fragment can not produce the whole. The adult 

 individual is more complex than the fertilized egg, yet 

 the egg has greater potentialities than any tissue-cell has. 

 Regeneration depends on the presence of embryonic, i. e., 

 non-tissue, cells lying latent amidst the tissue cells. The 

 greater complexity of the adult as a whole over tlie egg 

 does not hold for a given tissue cell— that is less complex 

 than the egg. The complexity of the adult is due to the 

 fact that there are in the body many kinds of tissue eel Is, 

 each simple, while the egg is just one kind of cell— but 

 very complex in constitution. Similarly, we may infer 

 that while the vast number of kinds of germ plasms in 

 the higher organisms, differentiated in respect to their 

 chromomeres, contrasts with the condition in the pro- 

 tista, it is probable that each chromomere of the protista 

 is composed of much more complex organic molecules or 

 molecule complexes. 



2. Another mass of evidence for this theory is sup- 

 plied by paleontology. By this science are offered ex- 

 tensive series showing: (a) the usual beginning of a new 

 character as a simple, often inconspicuous trait, (b) the 

 increase of variety in the course of evolution of a phylum 

 ending in great outburst of extreme and bizarre forms 

 immediately preceding the extinction of a phylogenetic 

 line; ( c ) the irreversibility of the process of evolution; 

 and (d) parallelism in evolution of allied lines. 



(a) The Simple Beginning of a Trait.-The early his- 

 tory of a number of spinose groups of species shows 

 (Beecher, 1898) that each group began its history in 



