MUSEUM OF COMPARATIVE ZOOLOGY. 87 



The rule (5) that aucestral rows contain fewer generations of indi- 

 viduals than lateral ones may perhaps receive a partial explanation from 

 the further fact (rule 6) that of the two rows starting from any axil the 

 aucestral branch will give rise to a greater total number of individuals 

 than the lateral one will in the same time. We should expect a less 

 rapid forward growth if the lateral growth is extremely vigorous. One 

 might also say that the intermediate rows had grown abnormally in 

 length, since that is the direction in which there is most room. 



The reason why the ancestral branches in Bugula give rise to the 

 greater total number of individuals is, to my mind, because they are 

 marginal. In Crisia it is the lateral branches which are the most 

 prolific, and for the same reason. 



The existence of the 7th rule in mat-like species is a mechanical 

 necessity ; in the phytoid species, like Bugula and Crisia, it must be ac- 

 counted for on another ground ; namely, on the relations of food supply 

 to demand, — on the deterrent effects of overcrowding. And this, to my 

 mind, is the key to the significance of the 4th, 5th, 6th, and 7th rules. 

 The form of the stock is determined by the same law which has deter- 

 mined the form of the individuals, — the struggle for existence and the 

 survival of the fittest, — the fittest in the present case being those which 

 are most advantageously placed with reference to food supply. Abundant 

 food supply has made possible the rapid production of lateral individuals 

 at the margin, and less abundant food supply has retarded such produc- 

 tion in the middle. Therefore has lateral budding occurred more rapidly 

 at the margin ; therefore has the number of individuals produced at the 

 margin been greatest ; therefore have the median rows grown in length 

 only with great rapidity ; therefore has the distance between adjacent 

 rows of individuals in phytoid stocks remained constant. 



Many observations on different groups of animals agree in demonstrat- 

 ing a relation between rapidity of the budding or fission process and food 

 supply. Thus Zoja ('90, pp. 25-27) has shown for Hydra, and Zacha- 

 rias ('86, p. 274) and von Wagner ^'90, p. 360) for Turbellarians, that 

 abundant food supply results in an acceleration of the processes of 

 non-sexual reproduction, and Braem ('90, p. 24) has shown that bud- 

 ding in Cristatella proceeds less actively during the late fall. This 

 diminution in activity has been attributed by Braem to diminished 

 temperature ; but we know also that this period is one of scarcity of tlie 

 small fresh water organisms upon which the fresh water Bryozoa live 

 (cf. Parker, '90, pp. 597-600), and this fact also must be considered as 

 having an important influence in this case. 



