Beagle: Journey's End following On the Origin of Species, revolutionary work (6)
I believe the answer is:
snoopy
'beagle' is the definition.
(I've seen this before)
'journey's end following on the origin of species revolutionary work' is the wordplay.
'end' indicates one should take the final letters.
'following' says to put letters next to each other.
'the origin of' indicates taking the first letters.
'revolutionary' says the letters should be written in reverse (letters revolve around).
'work' becomes 'op' (abbreviation for opus).
The final letter of 'journey' is 'y'.
The first letter of 'species' is 's'.
'on'+'s'='ons'
'ons' in reverse letter order is 'sno'.
'sno'+'op'='snoop'
'y' after 'snoop' is 'SNOOPY'.
(Other definitions for snoopy that I've seen before include "Prying, as in Peanuts" , "Charlie Brown's canine companion" , "Curious" , "Brown dog's" , "Cartoon character" .)