vii:\\s i»i NAii i;k itiNi |{a.>%ti.;ij. 21 



I*()|)uljir (»|iiiii(»n iin-lincs t(» llic Ixlict' that physiolotry is 

 tlu' bet^t ^uidc to the classiiicat ioii ot aiiiiiial> 



Science iirDclaiin- that |>liysii>l();xy i^ '''(• iii(»>l delusive 

 Uiient for the (lisro\rry >>{' tlic true i-clati')iiv of (.I'^aniztMl 

 beings. 



Ill popular loiiic, a^aiii, ilcilihl ion isilic |)i-iiiic clciiu'iil 

 involved, it is assumed (hat eei'taiu thiuiis arc, aud tVoni 

 this assuuij)tion tlie reasoner proceeds to aj)proxiniate 

 successively and without sufficient i-eason. the forms thai 

 ixvc ])rescnted for examination, 



111 science, on the other hand, it is iiuluetion wliich is 

 principally employed in mental processes; for example, we 

 take a series of forms, compare tliem toj^ether, contrast all 

 the elements of the several parts, and are jj^uided by the 

 detailed cumulative evidence of the marshalled facts. 



The people use a noiiieiiclature based on adaptations of 

 parts for similar |»urposes, as the feet for walkino;, th(> win^rs 

 for llit;ht. the tins for swimming. 



Science, while availiiiii itself of the ])opular nomenclature 

 to express one class of relatioiishii) — analogies — is compelled 

 to resort to one of its own coin in." to express another set of 

 ri'lationships — homologies. 



Now both of the words I have just used are constantly 

 employed in scientific works, and all of you who have read 

 such to any extent must have often come across them. But 

 I trust to be i)ardoned if I explain their meaning, for it is 

 impos.sible to begin to understand the problems of biology 

 unle.<s their -ignitication is clearly apprehenderl. 



Analogy is the adaptation of jiarts to similar functions but 

 tlujse parts may have no relation to each other nor be devel- 

 oped f.iom the .same ]»artsof the body. The gills of the fish 

 are analogous to the lungs of the whale, inasmuch a- both 

 are subservient to the otlice of resi»iratioii ; but they are 

 not homologous. The tail lin of the lish and the tail fin 

 of the whale are also analogous, but they are not homolo- 

 gous. 



Homology is similarity or rc|)ctition of structure, indcjien- 

 dent of the uses of the parts. The ventral tins of the lish. 



