The IGI records are far from perfect.
However, the birth records you have found are "extracted" - meaning the register has been seen and transcribed.
"Patron Submissions" are sometimes of dubious source.
1824 is before the start of civil registration in 1837.
So there is no birth certificate to be had.
Unless you want to see the actual register for yourself, the IGI is as good as it gets.
This would be the marriage:
Marriages Sep 1848
Higman John Stoke Damerel vol 9 p 530
SELDON Amelia Stoke Damerel vol 9 p 530
The marriage certificate should be available.
It should give the father's names, and names of witnesses.
You can find how to purchase here:
http://www.gro.gov.uk/gro/content/certificate/index.asp#0and
http://www.gro.gov.uk/gro/content/certificates/faqs.aspThe index reference you will need is vol 9 page 530 as above.
I don't know what else you have, but you have traced them back to Devon.
The names are unusual enough that tacing back should be reasonably easy.
First step might be a request for a lookup in the 1841 census.
Question: do you have evidence that this is your family, or did you just find a likely name on IGI?
Why they left England?
There were various Assisted Immigrant schemes at this time.
But few people would just up and move halfway round the globe.
You will often find that they joined family or friends already settled.
If you know where they settled, look for other families from Devon.
Quick look at 1851 census does not find the Higman parents/siblings.
Perhaps they went too. At the same time or later?
Regards CP