In the episode "The Green Muse", Dr Ogden and Detective Murdoch end up making out and they nearly go all the way. Julia stops it just on time with asking William if he has any "precautions" or prophylactics. Obviously, he doesn't as he didn't know he would be needing any, not to mention that contraception methods are illegal. However, it's later revealed that Dr Ogden is barren and she couldn't conceive anyway, so why would she ask for condoms? The brilliance is in her knowing that sex would be a step much further out of their comfort zone. Even though they are both ahead of their time, sexual relationship before wedding would be too much or going too quickly.
And as a pathologist, she's probably seen the results of sexually transmitted diseases like Syphilis. (Fridge Horror — the prophylactics of that time would have provided little to no protection.)
Alternately, Dr. Ogden may have suspected she could still conceive a child, but never successfully carry one to term. Having to live through the pain and grave danger of losing a pregnancy isn't something she'd be willing to risk, particularly if she's seen the results of a fatal miscarriage or two on her autopsy table. (Indeed, later seasons would see her and Murdoch's attempts to have a child end in miscarriages.)
In the episode "Murdoch on the Corner", Higgins and Crabtree are walking the beat, when they find a wallet with money in it. Higgins wants to keep it, but Crabtree convinces him to return it to the rightful owner. The wallet was revealed to be one of the many set ups created by the episode's murderer, and anyone who kept the money ended up dead. So, if Crabtree wasn't there...
Bobby and John Brackenreid are just the right age to end up fighting in World War One. Watts is also young enough to enroll/be drafted, but there is canon confirmation he lives into the early 20s at least.
In "Still Waters," the episode opens with Murdoch demonstrating a proto-lie detector that he's invented. Lie detectors have since largely been discredited due to poor accuracy, but not after numerous false convictions and acquittals...which Murdoch unwittingly helped happen.
