2i8 Cataract [ch. 



their descent. But in dealing with this evidence difficulty 

 arises from the fact that the age at which the cataract 

 appears is liable to great variation and consequently the 

 negative record, that a person was not affected, is of doubt- 

 ful value, for the affection might have been due to appear 

 later in life. As we should expect, however, it is in respect 

 of cataracts of this kind that transmission through the 

 unaffected is recorded with considerable frequency"*. 



There are several examples of families showing even 

 congenital cataract in several members where the affection 

 seems to have arisen de novo, no history of cataract being 

 discoverable in the parentage. With regard to the sub- 

 sequent transmission of such cataracts no very definite 

 information at present exists. 



As there is obvious doubt of the completeness of the 

 records in many instances it is not possible to give numeri- 

 cal data with exactitude. A fair idea of the general run of 

 the numbers may be gathered from the following list of the 

 more complete families. These are somewhat arbitrarily 

 selected from Nettleship's collection, but all the more 

 extensive families are included t. Some of the more 

 aberrant numbers even in these more perfect records are 

 almost certainly due to the sources of error named above. 



* It should nevertheless be remarked that the appearance of the 

 cataract at earlier ages in successive generations — "anticipation," as 

 Nettleship calls it — has been observed with some frequency. 



t Except No. 98, Green's family, where the cataract appeared at 

 various ages, sometimes not till advanced life. The numbers in it are 

 18 aflfected, and 37 unaffected. Transmission here occurred twice through 

 the unaffected. Since this table was made Nettleship has published some 

 new and important pedigrees (210). One of these gives a izmAy froin two 

 DR parents with senile cataract. Up to time of writing their children 

 were 7 affected, 2 unaffected, and i died at 45 with sight good. 



