1916] CURRENT LITERATURE 321 
way we have established a region of relatively high metabolic rate in the 
undifferentiated protoplasmic mass. From this point of high metabolic rate 
there is a radiation of dynamic change over the surface or through the mass 
of protoplasm. It is a transmissive, not a transportive, change, and in some 
way involves an increase in metabolic activity as it spreads over or through 
the protoplasm. In other words, there is a wave of increased chemical activity 
spreading away from the point of impinging stimulus 
This metabolic change cannot be transmitted “i indefinite distances, for 
the intensity of the metabolic change decreases with transmission. is 
transmission-decrement partly determines the limits of transmission. Other 
factors upon which the limit of transmission depends are the intensity of the 
original stimulus and the relative conductivity of the protoplasm. The limit 
of transmission determines the size limit of the individual, for parts that lie 
beyond the transmission limit remain physiologically isolated. 
Momentary action of the stimulus may lead only to a temporary increase 
in metabolic rate at the point of origin, but prolonged or frequent excitation 
may lead to more or less permanent changes in the protoplasm, chief among 
which is an increased irritability. The greatest increase in irritability occurs 
in the region of highest metabolic rate, which is the region of stimulus, and is 
progressively less in regions of lower and lower metabolic rates, reaching zero 
at the limit of transmission. Thus we may have established a gradient in the 
condition of the protoplasm, an irritability gradient, which constitutes the 
substratum for a persistent metabolic gradient independent of the local 
stimulus. For as soon as a persistent irritability gradient is established, uni- 
form stimulation will be followed by a graded metabolism dependent upon the 
degree of irritability of each axial level. Such persistent metabolic gradients 
are looked upon as the starting-point of organization. They are factors in 
determining the direction of growth and differentiation, and are therefore the 
basis of the space and time relations of development. 
The phenomenon of dominance of one part or region over other regions 
of the organism during development is simply a result of these differences in 
metabolic rates, levels of high rate dominating and controlling those of lower 
rate, and the interrelations of the parts of the organism, depending thus upon 
differences of metabolic rate, constitute the foundation of the unity and order 
exhibited by it. From this standpoint the organic individual is viewed as 
consisting of one or more gradients in a protoplasmic mass of specific physico- 
chemical constitution; and the process of becoming an individual is merely 
the process of establishing the necessary gradients. Such gradients, which 
arise de novo in response to external conditions, and cannot originate in any 
other way, may nevertheless become hereditary and persist through many 
Senerations when once establish 
This dynamic theory is distingniabed by its satisfactory interpretation of 
organic unity in terms of differences in rate of metabolism and of transmi ted 
change, rather than in terms of hypothetical organization and transportation 
