MUSEUM OF COMPARATIVE ZOOLOGY. 87 
The rule (5) that ancestral 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 
ancestral 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 oceurred 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 scareity of the 
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. 
