Canadian military organization followed the example set by the British regimental system with some exceptions. The first world war is an example of some of those exceptions. When Britain declared war on Germany on August 4, 1914, Canada as a self-governing dominion in the British Empire was constitutionally also at war. But there was a problem. The Militia Act did not give the Canadian Government legal authority to send serving members of the non-permanent militia out of the country to fight. The solution was to send militia members into numbered battalions for overseas service. Thus infantry found not in their regimental units but as a numbered battalion. Needless to say, there were exceptions such as cavalry regiments. That meant that usually the numbered battalions were recruited geographically. Again there were exceptions; the St. Thomas, Ontario, battalion (the 91st. Battalion) was recruited in 1915. But when they arrived in England in 1916 the battalion was broken up as reinforcements for other battalions in France.
Because of the geographic nature of the recruitment a genealogist should first look for the battalion that was recruited in the city or town where the ancestor lived. For example, the battalions recruited in South-western Ontario(that is to say from roughly Kitchener to Windsor) were organized as the following:
1st. Infantry Brigade(1914)
1st. Battalion (Western Ontario) recruited from:
1st Hussars
7th Regiment (Fusiliers)
21st Regiment (Essex Fusiliers)
22nd Regiment (The Oxford Rifles)
23rd Regiment (The Northern Fusiliers)
24th Kent Regiment
25th Regiment
28th Perth Regiment
29th Regiment (Highland Light Infantry of Canada)
77th Wentworth Regiment
2nd Battalion (Eastern Ontario)
9th Mississauga Horse
The Governor General's Foot Guards
14th Regiment (The Princess Of Wales' Own Rifles)
15th Regiment (Argyll Light Infantry)
16th Prince Edward Regiment
34th Ontario Regiment
42nd Lanark and Renfrew Regiment
43rd Regiment (The Duke of Cornwall's Own Rifles)
49th Regiment (Hastings Rifles)
59th Stormont and Glengarry Regiment
3rd Battalion (Toronto Regiment)
Governor General's Body Guard
2nd Regiment (Queen's Own Rifles of Canada)
10th Regiment (Royal Grenadiers)
13th Royal Regiment
4th Battalion (Central Ontario)
12th Regiment (York Rangers)
19th Lincoln Regiment
20th Regiment (Halton Rifles)
35th Regiment (Simcoe Foresters)
36th Peel Regiment
44th Lincoln and Welland Regiment
The 2nd. Brigade was largely recruited from Western Canada. The 3rd. Brigade was recruited from Eastern Ontario, Quebec, and the Maritimes.
Its somewhat complicated as there are the exceptions that I mentioned above. There are a few histories of the Battalions on-line. To find them google the Battalion; but do not be disappointed if you find little that you can find to use. The Matrix Project(
http://cefresearch.com/matrix/index.html) has a great deal of information but be warned that you need patience and some research skill to work your way through the information. Other paces to start are the Canadian Military Heritage Project(
http://www.rootsweb.ancestry.com/~canmil/), Library and Archives Canada(
http://www.collectionscanada.gc.ca/databases/cef/index-e.html), and the Canadian Great War Project(
http://www.canadiangreatwarproject.com/).
For assistance do not be afraid to ask.