aa ON THE TENDENCY OF VARIETIES, ETC.: 
by minute steps, in various directions, but always 
checked and balanced by the necessary conditions, 
subject to which alone existence can be preserved, 
may, it is believed, be followed out so as to agree 
with all the phenomena presented by organized be- 
ings, their extinction and succession in past ages, 
and all the extraordinary modifications of form, in- 
stinct and habits which they exhibit. 
