In isolated communities, on islands, among the mountains, families still remain in the same locality for generations, and people are born, marry and die with the same environment.  Their circle of acquaintance is very limited, and cousin marriage is therefore more frequent.  If we exclude such places, and consider only the more progressive American communities, it is entirely possible that the proportion of first cousin marriages would fall almost if not quite to .5 per cent.  So that the estimate of Dr. Dean for Iowa may not be far out of the way.

Even for England Mr. Darwin’s figures are probably much too large.  Applying the corrected formula his table becomes: 

TABLE VI.
----------------------------------------------------
|Number     |Per cent of|Per cent of
1872.          |marriages  |same-name  |first cousin
|registered.|marriages. |marriages.
----------------------------------------------------
London,        |           |           |
Metropolitan   |           |           |
Districts      | 33,155    |   .55     |  .73
Urban Districts| 22,346    |   .71     |  .95
Rural Districts| 13,391    |   .79     | 1.05
----------------------------------------------------
Total    | 68,892    |   .64     |  .85[A]
----------------------------------------------------
[A] Cf.  Mulhall, .75 per cent, supra, p. 18.

In regard to the frequency of marriage between kin more distant than first cousins figures are still more difficult to obtain.  The distribution of 514 cases of consanguineous marriage from genealogies was as follows: 

TABLE VII.
------------------------------------------------------------

---------
| First | 1-1/2 |Second | 2-1/2 | Third |Distant|
|cousins|cousins|cousins|cousins|cousins|cousins|Total
------------------------------------------------------------

---------
Same-name     |  70   |   24  |  49   |  19   |  20   |  26   | 208
Different-name|  96   |   30  |  58   |  22   |  37   |  62   | 305
------------------------------------------------------------

---------
Total       | 166   |   54  | 107   |  41   |  57   |  88   | 513
------------------------------------------------------------

---------

Obviously this cannot be taken as typical of the actual distribution of consanguineous marriages, since the more distant the degree, the more difficult it is to determine the relationship.  However it is very evident that the coefficient of attraction is at its maximum between first cousins, and probably there are actually more marriages between first cousins than between those of any other recognized degree of consanguinity.  But the two degrees of 1-1/2 cousins and second cousins taken together probably number more intermarriages than first cousins alone.  Allowing four children to a family, three of whom marry and have families, the actual number of cousins a person would have on each degree would be:  First, 16; 1-1/2, 80; Second, 96; 2-1/2, 480; Third, 576; Fourth, 3,456.  The matter is usually complicated by double relationships, but it will readily be seen that the consanguineal attraction would hardly be perceptible beyond the degree of third cousins.[27]