31 
been ascertained and recorded, at intervals of a few years, for 
more than a century, by a systematic method of counting the 
contents of a certain number of sample dredgings. From a 
Danish book on the subject (Der Danske Osterbanker, by H. 
Kroyer, Kjobenhaven, 1839), and from other sources, he ob- 
tains the following series of ratios : 
In 1730 there were 486 lialf-grown oysters to every 1,000 full-grown ones. 
“ 1734 “ 
310 
44 
14 
44 
1,000 
44 
“ 1740 “ 
418 
44 
44 
44 
1,000 
44 
“ 175G “ 
490 
44 
44 
44 
1,000 
44 
“ 1775 “ 
484 
(4 
44 
44 
1,000 
“ 1709 
307 
44 
44 
44 
1,000 
44 
“ 1819 “ 
388 
44 
44 
“ 
1,000 
44 
“ 1830 “ 
417 
44 
44 
44 
1,000 
“ 1839 “ 
440 
“ 
44 
“ 
1,000 
44 
“ 1852 
473 
44 
44 
1,000 
44 
or an average of 421.2 to 1 ,000, or 42.13 per cent, of young. 
The uniformity of the ratio between the halt-grown and the 
adult oysters, through a period of more than one hundred 
years, is very remarkable. In no case was there less than 
30 per cent, of young or more than 50 per cent., and the 
average of 42 per ccift. is very closely followed After an oys- 
ter has become half-grown its dangers are very few, and this 
number — 42 — probably gives with sufficient accuracy the num- 
ber of oysters which grow up each year for each 100 adults. 
Although the number - of young which are born each year is 
great beyond computation, those which survive are less than 
half as numerous as the adults. 
Mobius has estimated the number of adults which spawm 
each year, and multiplying this number by the average num- 
ber of eggs laid by each, and dividing by the number which 
grow up, he reaches the conclusion that each oyster which is 
born has of a chance of reaching maturity. In the case 
of the American oyster, the number of eggs is very much 
greater, and each one’s chance of survival is accordingly very 
much less, and it is evident that the great fertility of 
the oyster will not protect a bed from destruction by exces- 
sive dredging, for while the young spat frequently cover the 
