28 /. D. Dana on the Classification of Crustacea. 



Y. In the opposite extreme of organic beings, the vegetable 

 kingdom, the same principle is illustrated. Plants may be so 

 minute as to have free motion and activity, as in animals. The 

 spores of certain Algae are known to have powers of locomotion, 

 and some so-called Infusoria, are now admitted to belong to the 

 vegetable kingdom. These are examples of locomotive plants. 

 Now, ordinary plants, like Cirripeds, are examples of sedentary 

 species, that have outgrown the limits of activity. The life-sys- 

 tem of a plant, is in fact sufficient in power to give locomotion 

 only to the minute plant-individuals alluded to ; and infusorial 

 species of plants retain it, as long as they live. But when, as in 

 the Algse, vegetative growth proceeds in the enlargement of the 

 minute infusorial spore, it immediately outgrows its activity, and 

 becomes a sedentary plant. In most other plants, the seed have 

 never the minute size which admits of motion. 



The mean size of the Entomostracan type was stated to be one 

 line; of the Eotatorial type, one-sixth of a line; and we may add, 

 that the mean size of the Plant type — understanding by this, as 

 in other cases, the mean size admitting of the highest activity — 

 if deduced from the size of plant-infusoria, would be about one- 

 sixtieth of a line. 



We observe, that the smallest size of the perfect Macroura 

 (first type) is very nearly the mean size as to length of the ani- 

 mals of the second type. So also, the smallest size of the perfect 

 animal of the second type (Tetradecapoda) is very nearly the 

 mean size of the most perfect animals of the third type ; and the 

 smallest size of the perfect animal of the third type is nearly the 

 largest size in the fifth type. 



In order to compare allied animals of different sizes, it should 

 be noted, that while there is some foundation for the conclusion, 

 that under certain limitations, size is a mark of grade, rapidity 

 of movement or action should also be considered ; and the more 

 proper comparison would be between multiples of size and 

 activity. This deduction, is, however, true only in the most gen- 

 eral sense, and rather between species of allied groups than those 

 of different types. We may occasionally find something like an 

 exemplification of the law among bipeds, ludicrous though the 

 idea may be. 



YI. We observe with regard to the passage in Crustacea to 

 inferior grades under a given type, that there are two methods 

 by which it takes place. 



1. A diminution of centralization, leading to an enlargement of 

 the circumference or sphere of growth at the expense of con- 

 centration, as in the elongation of the antennas and a transfer of 

 the maxiHipeds to the foot series, the elongation of the abdomen 

 and abdominal appendages, etc. 



